title: A prospective cohort study of a woman's own gestational age and her fecundability.
abstract: OBJECTIVE
  What is the magnitude of the association between a woman's gestational age at her own birth and her fecundability (cycle-specific probability of conception)?
  
  
  CONCLUSIONS
  We found a 62% decrease in fecundability among women born <34 weeks of gestation relative to women born at 37-41 weeks of gestation, whereas there were few differences in fecundability among women born at later gestational ages.
  
  
  BACKGROUND
  One study, using retrospectively collected data on time-to-pregnancy (TTP), and self-reported data on gestational age, found a prolonged TTP among women born <37 gestational weeks (preterm) and with a birthweight ≤1500 g. Other studies of women's gestational age at birth and subsequent fertility, based on data from national birth registries, have reported a reduced probability of giving birth among women born <32 weeks of gestation.
  
  
  METHODS
  We used data from a prospective cohort study of Danish pregnancy planners ('Snart-Gravid'), enrolled during 2007-2011 and followed until 2012. In all, 2814 women were enrolled in our study, of which 2569 had complete follow-up.
  
  
  METHODS
  Women eligible to participate were 18-40 years old at study entry, in a relationship with a male partner, and attempting to conceive. Participants completed a baseline questionnaire and up to six follow-up questionnaires until the report of pregnancy, discontinuation of pregnancy attempts, beginning of fertility treatment, loss to follow-up or end of study observation after 12 months.
  
  
  RESULTS
  Among women born <34 gestational weeks, the cumulative probability of conception was 12, 28 and 48% within 3, 6 and 12 cycles, respectively. Among women born at 37-41 weeks of gestation, cumulative probability of conception was 47, 67 and 84% within 3, 6 and 12 cycles, respectively. Relative to women born at 37-41 weeks' gestation, women born <34 weeks had decreased fecundability (fecundability ratio (FR) 0.38, 95% confidence interval (CI): 0.17-0.82). Our data did not suggest reduced fecundability among women born at 34-36 weeks of gestation or at ≥42 weeks of gestation (FR 1.03, 95% CI: 0.80-1.34, and FR 1.13, 95% CI: 0.96-1.33, respectively).
  
  
  CONCLUSIONS
  Data on gestational age, obtained from the Danish Medical Birth Registry, were more likely to be based on date of last menstrual period than early ultrasound examination, possibly leading to an overestimation of gestational age at birth. Such overestimation, however, would not explain the decrease in fecundability observed among women born <34 gestational weeks. Another limitation is that the proportion of women born before 34 weeks of gestation was low in our study population, which reduced the precision of the estimates.
  
  
  CONCLUSIONS
  By using prospective data on TTP, our study elaborates on previous reports of impaired fertility among women born preterm, suggesting that women born <34 weeks of gestation have reduced fecundability.
  
  
  BACKGROUND
  The study was supported by the National Institute of Child Health and Human Development (R21-050264), the Danish Medical Research Council (271-07-0338), and the Health Research Fund of Central Denmark Region (1-01-72-84-10). The authors have no competing interests to declare.
authors: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Allé 43-45, 8200 Aarhus N, Denmark cwni@clin.au.dk.
authors: C
authors: C
authors: Wildenschild
authors: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Allé 43-45, 8200 Aarhus N, Denmark.
authors: A H
authors: AH
authors: Riis
authors: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Allé 43-45, 8200 Aarhus N, Denmark.
authors: V
authors: V
authors: Ehrenstein
authors: Department of Epidemiology, Boston University School of Public Health, 715 Albany Street, Boston, MA 617857, USA.
authors: E E
authors: EE
authors: Hatch
authors: Department of Epidemiology, Boston University School of Public Health, 715 Albany Street, Boston, MA 617857, USA Slone Epidemiology Center, Boston University, 1010 Commonwealth Ave, 4th Floor, Boston, MA 02215, USA.
authors: L A
authors: LA
authors: Wise
authors: Department of Epidemiology, Boston University School of Public Health, 715 Albany Street, Boston, MA 617857, USA RTI Health Solutions, 200 Park Offices Drive, Research Triangle Park, NC 27709, USA.
authors: K J
authors: KJ
authors: Rothman
authors: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Allé 43-45, 8200 Aarhus N, Denmark Department of Epidemiology, Boston University School of Public Health, 715 Albany Street, Boston, MA 617857, USA.
authors: H T
authors: HT
authors: Sørensen
authors: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Allé 43-45, 8200 Aarhus N, Denmark.
authors: E M
authors: EM
authors: Mikkelsen
mesh_terms: D001724:Birth Weight; D003718:Denmark; D005260:Female; D005298:Fertility; D005306:Fertilization; D005865:Gestational Age; D006801:Humans; D007247:Infertility, Female / Q000473:pathology; D011247:Pregnancy; D011336:Probability; D011446:Prospective Studies; D012042:Registries; D012189:Retrospective Studies; D061685:Time-to-Pregnancy
keywords: cohort study; fecundability; female infertility; gestational age; preterm birth