This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow E-mail this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My File Cabinet
Right arrow Download to citation manager
Right arrowRequest Permissions
Citing Articles
Right arrow Citing Articles via CrossRef
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Smith, G. C. S.
Right arrow Articles by Dobbie, R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Smith, G. C. S.
Right arrow Articles by Dobbie, R.
Related Collections
Right arrow Premature & Newborn
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Facebook   Add to Reddit   Add to Technorati   Add to Twitter  
What's this?
PEDIATRICS Vol. 111 No. 6 June 2003, pp. 1367-1371

Risk of Sudden Infant Death Syndrome and Week of Gestation of Term Birth

Gordon C. S. Smith, MD, PhD*, Jill P. Pell, MD{ddagger} and Richard Dobbie, BSc§

* Departments of Obstetrics and Gynaecology, Cambridge University, Cambridge, United Kingdom
{ddagger} Department of Public Health, Greater Glasgow Health Board, Glasgow, Scotland
§ Information and Statistics Division, Common Services Agency, Edinburgh, Scotland


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Objective. We sought to determine if the risk of sudden infant death syndrome (SIDS) varied according to week of delivery at term among elective and nonelective births.

Design. Retrospective cohort study.

Participants. All single infants live born between 37 and 42 weeks gestation in Scotland between 1992 and 1995 documented in the Scottish Morbidity Record.

Outcome. Death in the first year of life where SIDS was in the principal position on the death certificate.

Results. There were 202 622 eligible births and 119 deaths attributed to SIDS. Among infants delivered electively, there was no significant association between risk of SIDS and week of delivery. Among those delivered nonelectively, the risk of SIDS declined significantly with each week of gestation (odds ratio .72, 95% confidence interval .60–.86). This trend was only minimally attenuated by adjustment for maternal age, parity, smoking and socioeconomic deprivation category, infant sex, Apgar score, mode of delivery, and birth weight decile (adjusted odds ratio .78, 95% confidence interval .65–.93).

Conclusions. We hypothesize that early spontaneous labor at term and SIDS may be linked because of a common association with suboptimal intrauterine environment.

Key Words: pregnancy • gestational age • labor • sudden infant death syndrome • smoking

Abbreviations: CI, confidence interval • HPAA, hypothalamopituitary adrenal axis • SIDS, sudden infant death syndrome


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Previous epidemiologic studies on the etiology of sudden infant death syndrome (SIDS) have identified a number of obstetric risk factors, such as maternal cigarette smoking in pregnancy, low birth weight, low maternal age and high parity, and a number of postnatal environmental factors, such as socioeconomic deprivation, parental cigarette smoking, sleeping position, and bedding material.1–5 Identifying infants at increased risk of SIDS allows education and monitoring to be targeted toward high-risk families. Identification of modifiable environmental risk factors led to public information campaigns which have been followed by a dramatic decrease in the incidence of SIDS.4,6 Despite these improvements, the pathophysiology of SIDS is still unclear and it remains the most common cause of postneonatal death in the first year of life.7

Moderate and severe preterm birth are associated with an increased risk of SIDS and account for a significant etiologic fraction of all SIDS deaths.8 We have recently shown that early pregnancy levels of circulating placental proteins are associated both with spontaneous preterm birth9 and with the earlier onset of labor at term.10 We hypothesize that in some pregnancies the early onset of labor at term reflects a suboptimal intrauterine environment. The aim of the present study was to determine if the risk of SIDS varied with the timing of spontaneous labor among infants born at term.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Patient Selection
The Scottish Morbidity Record collects information on clinical and demographic characteristics and outcomes for all women discharged from Scottish maternity hospitals. The register is subjected to regular quality assurance checks and has been >99% complete since the late 1970s.11 Data were extracted on all single live born infants delivered between 37 and 42 weeks gestation in Scotland between 1992 and 1995. Scottish Morbidity Record records were linked to the death database held by the Registrar General for Scotland’s Office to determine the date and principal cause of death of those infants who died. We excluded from both the univariate and multivariate analyses those infants with missing values for any of the covariates of interest and those infants who died in their first year of life from a cause other than SIDS.

Definitions
SIDS was defined as the death of an infant for whom the principal cause of death on the death certificate was coded as 798.0 using the International Classification of Diseases, Ninth Revision. Over the period studied, a diagnosis of SIDS could only be written on a death certificate in Scotland following thorough investigation of the circumstances of the death including a postmortem examination performed by an approved pediatric pathologist.

Gestational age has been confirmed by ultrasound scan in the first half of pregnancy in >95% of pregnancies in the United Kingdom from the early 1990s.12 Gestational age at birth was defined as completed weeks of gestation on the basis of the estimated date of delivery in each woman’s clinical record and standard national criteria exist for the estimation of date of delivery using menstrual and ultrasound data.13 Smoking status was defined as the smoking status of the woman at the time of first attendance for antenatal care. Maternal age was defined as the age of the mother at the time of delivery. Birth weight was categorized into sex and gestational age-specific deciles, as previously described in detail.14 Elective delivery was defined as delivery by planned cesarean section or delivery by any mode following induction of labor. Postal code of residence was used to derive Carstairs socioeconomic deprivation categories.15 These are based on 1991 Census data on car ownership, unemployment, overcrowding, and social class within postal code sectors of residence which contain, on average, ~1600 residents.

Statistical Analyses
Continuous variables were summarized by the median and interquartile range, and comparisons between groups were performed using the Mann-Whitney U test. Correlation was tested using Spearman’s {rho}. Univariate comparisons of dichotomous data were performed using the {chi}2 test. Ordinal data were compared using the {chi}2 test for trend. The P values for all hypothesis tests were 2-sided. Crude and adjusted odds ratios were obtained using logistic regression analyses. Maternal age, height, socioeconomic deprivation score and mode of delivery, and the offspring’s birth weight decile, Apgar score, and sex were categorized and treated as dummy variables. Week of gestation at term was treated as a continuous variable and linearity or nonlinearity (in the log odds scale) was assessed using fractional polynomials.16 The possible effect of deaths attributed to other causes was taken into account using a proportional hazards model, where age was the time scale, death attributed to SIDS was the event, deaths in the first year of life attributed to causes other than SIDS were treated as censored, as were all survivors at 1 year of life. Regression techniques employed robust standard errors to allow for dependence within individuals using a maternal identifier. The statistical significance of interaction terms was assessed using the likelihood ratio test and significance assumed at P < .01. The goodness of fit of models was assessed using the Hosmer and Lemeshow17 test based on deciles of probability. All statistical analyses were performed using the Stata software package (Stata Corporation, College Station, TX), version 7.0.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Between 1992–1995, there were 241 846 single live births in Scotland. Gestational age at birth was missing in 666 (.3%), and recorded but outside the range of 37 to 42 weeks in 13 714 (5.7%). Among the remaining 227 466 births, there were missing values for parity in 155 (.1%), deprivation category in 741 (.3%), birth weight in 62 (<.1%), and maternal smoking in 23 626 (10.4%). Overall, 24 413 had 1 or more missing values, leaving 203 053 live births between 37 and 42 weeks gestation with complete data. Within this group, there were 431 (.2%) deaths before the first year of life attributed to causes other than SIDS, leaving a study group of 202 622. There were 119 deaths attributed to SIDS in the group which equated to a rate of 5.9 per 10 000 (95% confidence interval [CI] 4.9–7.0) for the study cohort. The proportion of infants dying from SIDS did not change significantly between 1992 and 1995 (P = .98, {chi}2 test for trend).

Table 1 compares the maternal demographic and obstetric characteristics of those infants who died from SIDS with those who did not (Table 1). On univariate analysis, the mothers of infants who died from SIDS were younger at delivery, of higher parity, were more likely to live in an area of high socioeconomic deprivation and were more likely to smoke. Infants who died from SIDS were of lower absolute birth weight, lower birth weight decile for sex and gestational age, were more likely to deliver at earlier weeks of gestation, and were more likely to be male. The median age of death attributed to SIDS was 78 days (interquartile range: 42–138 days). There was no association between gestation at delivery and age of death attributed to SIDS among infants born electively (Spearman’s {rho} = –.06, P = .75) or following spontaneous labor (Spearman’s {rho} = .19, P = .08).


View this table:
[in this window]
[in a new window]
  		TABLE 1. Comparison of Maternal, Demographic, and Obstetric Characteristics by Occurrence of SIDS

 
The relationship between the risk of SIDS and the week of gestation at delivery was then examined separately for elective and nonelective births. There was no significant trend between week of delivery at term and the risk of SIDS among infants delivered electively, whereas among infants delivered nonelectively the risk of SIDS decreased with increasing gestational age at delivery (Fig 1). The relationship between week of gestation and risk of SIDS was explored further using logistic regression. The odds ratio for each week of gestation at term for SIDS among nonelective births was .72 (95% CI .60–.86, P < .001) and for elective births was .90 (95% CI .70–1.15, P = .41). Addition of a polynomial term did not significantly improve either model (P = .38 and P = .36). Adjusting for potential confounders (maternal age, parity, deprivation category, mode of delivery, smoking status, birth weight decile, Apgar score, and sex) only slightly attenuated the association among nonelective births (adjusted odds ratio .78, 95% CI .65–.93) and minimally altered the odds ratio for week of gestation among elective births (adjusted odds ratio .88, 95% CI .68–1.13). The odds ratio of SIDS at each week of gestation among nonelective births is tabulated (Table 2).


Figure 1
View larger version (19K):
[in this window]
[in a new window]
  		Fig 1. Death rates for SIDS in relation to week of delivery among women delivered electively (N = 59 983) and nonelectively (N = 142 639). P value is the {chi}2 test for trend.

 

View this table:
[in this window]
[in a new window]
  		TABLE 2. Crude and Adjusted Odds Ratios for SIDS Associated With Week of Delivery at Term Among Infants Born Following the Onset of Spontaneous Labor

 
The association between week of gestation and the risk of SIDS among nonelective births was very similar when confined to women who booked before 20 weeks gestation (odds ratio .74, 95% CI .60–.91) or before 13 weeks gestation (odds ratio .69, 95% CI .50–.96). The association was also very similar when the risk of SIDS was calculated having taken into account infant deaths attributed to other causes using a proportional hazards model for both nonelective (hazard ratio .72, 95% CI .60–.86) and elective births (hazard ratio .90, 95% CI .70–1.15). There were no statistically significant interactions between week of delivery at term among either elective or nonelective births and the other factors in predicting the risk of SIDS (all P > .01). Goodness of fit tests were performed for logistic models and these were not statistically significant for either nonelective (P = .96) or elective births (P = .72).

There was limited information available on the indication for elective delivery. However, we could identify women who were delivered by planned cesarean section who had an infant presenting by the breech and women having planned repeat cesarean births. The incidence of SIDS in this group (4 of 9766, 4.1 per 10 000) was very similar to the other women delivered electively (29 of 50 217, 5.8 per 10 000; P = .52).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The factors predisposing to SIDS have been the subject of intense study for many years. The incidence of SIDS declined in many countries in the early 1990s as the importance of postnatal environmental factors, such as sleeping position and cigarette smoke, was recognized. The sharp decline in incidence took place between 1989–1991 in Scotland,6 and over the period of the present study the rate of SIDS was relatively stable. A number of obstetric factors are known to affect the risk of SIDS, including preterm birth. The main finding of the present study is that even within gestational ages regarded as term (from 37 weeks onwards), the risk of SIDS decreased progressively with advancing gestational age at delivery (Fig 1 and Table 2). This association was not explained by known risk factors for SIDS, such as maternal cigarette smoking, fetal sex, fetal growth, maternal age, parity, and socioeconomic deprivation. There was no comparable trend among infants delivered electively, and the differing patterns of association persisted after adjusting for mode of delivery. This suggests that the increased risk was not attributed to early delivery, but to the early onset of labor. We hypothesize that the factors which predispose to earlier onset of spontaneous labor at term may also predispose to SIDS.

It is possible that the lack of association between gestational age and risk of SIDS could be explained by bias. If infants born electively before 40 weeks had an increased risk of SIDS, then it is possible that those allowed to proceed to 40 weeks and beyond were selected and had a lower risk of SIDS. Therefore, a false relationship between gestational age and SIDS might be observed. However, the infants born electively before 40 weeks had a very similar risk of SIDS to infants born nonelectively at or after 40 weeks gestation. The excess of deaths occurred among infants nonelectively delivered at the earlier weeks of term. The rate of death in this group was higher than elective births at all gestational ages and spontaneous births at later weeks of gestation. It is possible that the association with early term birth reflected misclassification of gestational age and increased numbers of truly preterm infants at the earlier weeks of gestation at term. However, this is unlikely since confining the analysis to women who booked in the first trimester of pregnancy, when ultrasound estimation of gestational age is most accurate,13 tended to strengthen rather than weaken the association.

This study focused on the relationship between obstetric factors and the risk of SIDS. Although this analysis is retrospective, these data were collected during pregnancy and immediately following delivery and were thus prospective in relation to SIDS; this is one of the main strengths of our study design. However, we lacked information on some other factors which are thought to influence the risk of SIDS. For example, the method of infant feeding was unknown. It is possible that differences in successful breastfeeding in relation to week of delivery may account for some of the observed association. However, this would not explain the observed difference between elective and nonelective births. There were missing values for ~10% of the population, principally smoking status. This is unlikely to bias the analysis and the proportion with missing values is comparable to other national databases.18

A number of studies have demonstrated an increased risk of SIDS following preterm birth.4,19 Because SIDS is independently associated with admission to neonatal intensive care,19 it is difficult to determine if the association with preterm birth was a consequence of preterm delivery or was associated with the factors which led to the preterm delivery. Our study is the first to demonstrate an association between earlier onset of labor at term and the risk of SIDS in the infant. Although the biological determinants of the onset of labor at term are not yet fully understood, activation of the fetal hypothalamopituitary adrenal axis (HPAA) is thought to be a key physiologic event precipitating the process.20 The factors controlling the fetal HPAA are complex, and possibly include corticotrophin-releasing hormone from the placenta21 and suppression of the fetal HPAA by maternal steroids.22 However, a number of physiologic stresses activate the fetal HPAA.23 Previous studies have also shown an association between spontaneous preterm birth and intrauterine growth restriction.24–26 We hypothesize that early activation of the fetal HPAA and the resultant early onset of labor at term may reflect an adverse intrauterine environment. Consistent with this, we have recently demonstrated that low levels of placentally derived proteins in early pregnancy are associated with earlier onset of labor at term.10 We hypothesize that the association demonstrated between early onset of spontaneous labor and risk of SIDS may result from a suboptimal intrauterine environment predisposing to both events.

It has been hypothesized, on the basis of postmortem findings, that SIDS forms a continuum with unexplained death of the fetus in utero.27 Consistent with this, the conditions share a number of risk factors, such as maternal cigarette smoking during pregnancy,28 poor fetal growth,29 and male sex.14 However, the risk of unexplained fetal death increases with advancing gestational age,30,31 whereas the risk of SIDS declined (Table 2). Moreover, the risk of SIDS is lower among first pregnancies,4,19 whereas the risk of unexplained fetal death is higher among first pregnancies.31 We hypothesize that the differences in risk factors for SIDS and unexplained fetal death may be explained by the initiation of labor. We propose that if the suboptimal intrauterine environment prematurely initiates the endocrine cascade leading to parturition, the infant is live born but is at increased risk of subsequent SIDS. If the fetus is exposed to a suboptimal intrauterine environment but labor is not initiated, there is an increased risk of intrauterine death. We hypothesize that the lower risk of SIDS and higher risk of unexplained fetal death among nulliparous women may reflect a higher threshold for activation of the fetal HPAA to initiate labor. Consistent with this, the onset of spontaneous labor at term is later among nulliparous women.32


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The earlier onset of spontaneous labor at term is associated with an increased risk of subsequent SIDS. We hypothesize that this may reflect a common association with suboptimal intrauterine environment.


   FOOTNOTES
 
Received for publication Mar 13, 2002; Accepted Nov 18, 2002.

Reprint requests to (G.C.S.S.) Department of Obstetrics and Gynaecology, Cambridge University, Rosie Maternity Hospital, Cambridge, CB2 2QQ, United Kingdom. E-mail: gcss2{at}cam.ac.uk


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 

  1. Kraus JF, Greenland S, Bulterys M. Risk factors for sudden infant death syndrome in the US Collaborative Perinatal Project. Int J Epidemiol.1989; 18 :113 –120[Abstract/Free Full Text]
  2. Buck GM, Cookfair DL, Michalek AM, et al. Intrauterine growth retardation and risk of sudden infant death syndrome (SIDS). Am J Epidemiol.1989; 129 :874 –884[Abstract/Free Full Text]
  3. Malloy MH, Hoffman HJ. Prematurity, sudden infant death syndrome, and age of death. Pediatrics.1995; 96 :464 –471[Abstract/Free Full Text]
  4. Alm B, Norvenius SG, Wennergren G, et al. Changes in the epidemiology of sudden infant death syndrome in Sweden 1973–1996. Arch Dis Child.2001; 84 :24 –30[Abstract/Free Full Text]
  5. Sullivan FM, Barlow SM. Review of risk factors for sudden infant death syndrome. Paediatr Perinat Epidemiol.2001; 15 :144 –200[CrossRef][Web of Science][Medline]
  6. Arrundale J. Numbers of infant deaths in Scotland, with special reference to sudden infant death syndrome. Health Bull (Edinb).1993; 51 :106 –117
  7. Information and Statistics Division NHS Scotland. Scottish Perinatal and Infant Mortality and Morbity Report 2000 (available on-line at www.show.nhs.uk/isd). Edinburgh, United Kingdom: Common Services Agency; 2001
  8. Kramer MS, Demissie K, Yang H, Platt RW, Sauve R, Liston R. The contribution of mild and moderate preterm birth to infant mortality. Fetal and Infant Health Study Group of the Canadian Perinatal Surveillance System. JAMA.2000; 284 :843 –849[Abstract/Free Full Text]
  9. Smith GCS, Stenhouse EJ, Crossley JA, Aitken DA, Cameron AD, Connor JM. Early pregnancy levels of pregnancy-associated plasma protein a and the risk of intrauterine growth restriction, premature birth, preeclampsia, and stillbirth. J Clin Endocrinol Metab.2002; 87 :1762 –1767[Abstract/Free Full Text]
  10. Smith GCS, Stenhouse EJ, Crossley JA, Aitken DA, Cameron AD, Connor JM. Development: early-pregnancy origins of low birth weight. Nature.2002; 417 :916[CrossRef][Medline]
  11. Cole SK. Scottish maternity and neonatal records. In: Chalmers I, McIlwaine GM, eds. Perinatal Audit and Surveillance. London, United Kingdom: Royal College of Obstetricians and Gynaecologists; 1980:39–51
  12. Campbell S, Soothill PW. Detection and management of intrauterine growth retardation: a British approach. In: Chervenak FA, Isaacson GC, Campbell S, eds. Ultrasound in Obstetrics and Gynaecology, II. Boston, MA: Little Brown; 1993:1432–1435
  13. Evans E, Farrant P, Gowland M, McNay MB, Richards B. Clinical Applications of Ultrasonic Fetal Measurements. London, United Kingdom: British Medical Ultrasound Society/British Institute of Radiology; 1990
  14. Smith GCS. Sex, birth weight and the risk of stillbirth in Scotland, 1980–1996. Am J Epidemiol.2000; 151 :614 –619[Abstract/Free Full Text]
  15. McLoone P, Boddy FA. Deprivation and mortality in Scotland, 1981 and 1991. BMJ.1994; 309 :1465 –1470[Abstract/Free Full Text]
  16. Royston P, Altman DG. Regression using fractional polynomials of continuous covariates: parsimonious parametruc modeling. Appl Statistics.1994; 43 :429 –67[CrossRef]
  17. Hosmer DW, Lemeshow S. Applied Logistic Regression. New York, NY: John Wiley & Sons; 1989
  18. Olausson PM, Cnattingius S, Goldenberg RL. Determinants of poor pregnancy outcomes among teenagers in Sweden. Obstet Gynecol.1997; 89 :451 –457[CrossRef][Web of Science][Medline]
  19. Leach CE, Blair PS, Fleming PJ, et al. Epidemiology of SIDS and explained sudden infant deaths. CESDI SUDI Research Group. Pediatrics.1999; 104(4) . Available at: www.pediatrics.org/cgi/content/full/104/4/e43
  20. Mecenas CA, Giussani DA, Owiny JR, et al. Production of premature delivery in pregnant rhesus monkeys by androstenedione infusion. Nat Med.1996; 2 :443 –448[CrossRef][Web of Science][Medline]
  21. Challis JR, Matthews SG, Van MC, Ramirez MM. Current topic: the placental corticotrophin-releasing hormone-adrenocorticotrophin axis. Placenta.1995; 16 :481 –502[CrossRef][Web of Science][Medline]
  22. Pepe GJ, Albrecht ED. Actions of placental and fetal adrenal steroid hormones in primate pregnancy. Endocr Rev.1995; 16 :608 –648[Abstract/Free Full Text]
  23. Gardner DS, Fletcher AJ, Fowden AL, Giussani DA. Plasma adrenocorticotropin and cortisol concentrations during acute hypoxemia after a reversible period of adverse intrauterine conditions in the ovine fetus during late gestation. Endocrinology.2001; 142 :589 –598[Abstract/Free Full Text]
  24. Bukowski R, Gahn D, Denning J, Saade G. Impairment of growth in fetuses destined to deliver preterm. Am J Obstet Gynecol.2001; 185 :463 –467[CrossRef][Web of Science][Medline]
  25. Smith GCS, Smith MFS, McNay MB, Fleming JEE. First-trimester growth and the risk of low birth weight. N Engl J Med.1998; 339 :1817 –1822[Abstract/Free Full Text]
  26. Lackman F, Capewell V, Richardson B, daSilva O, Gagnon R. The risks of spontaneous preterm delivery and perinatal mortality in relation to size at birth according to fetal versus neonatal growth standards. Am J Obstet Gynecol.2001; 184 :946 –953[CrossRef][Web of Science][Medline]
  27. Walsh S, Mortimer G. Unexplained stillbirths and sudden infant death syndrome. Med Hypotheses.1995; 45 :73 –75[CrossRef][Web of Science][Medline]
  28. Raymond EG, Cnattingius S, Kiely JL. Effects of maternal age, parity, and smoking on the risk of stillbirth. Br J Obstet Gynaecol.1994; 101 :301 –306[Web of Science][Medline]
  29. Cnattingius S, Haglund B, Kramer MS. Differences in late fetal death rates in association with determinants of small for gestational age fetuses: population based cohort study. BMJ.1998; 316 :1483 –1487[Abstract/Free Full Text]
  30. Yudkin PL, Wood L, Redman CW. Risk of unexplained stillbirth at different gestational ages. Lancet.1987; 1 :1192 –1194[Web of Science][Medline]
  31. Smith GCS. Life-table analysis of the risk of perinatal death at term and post term in singleton pregnancies. Am J Obstet Gynecol.2001; 184 :489 –496[CrossRef][Web of Science][Medline]
  32. Smith GCS. Use of time to event analysis to estimate the normal duration of human pregnancy. Hum Reprod.2001; 16 :1497 –1500[Abstract/Free Full Text]
PEDIATRICS (ISSN 1098-4275). ©2003 by the American Academy of Pediatrics

Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Facebook Facebook   Add to Reddit Reddit   Add to Technorati Technorati   Add to Twitter Twitter    What's this?



This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow E-mail this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My File Cabinet
Right arrow Download to citation manager
Right arrowRequest Permissions
Citing Articles
Right arrow Citing Articles via CrossRef
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Smith, G. C. S.
Right arrow Articles by Dobbie, R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Smith, G. C. S.
Right arrow Articles by Dobbie, R.
Related Collections
Right arrow Premature & Newborn
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Facebook   Add to Reddit   Add to Technorati   Add to Twitter  
What's this?