title: Prediction of an estimated delivery date should take into account both the length of a previous pregnancy and the interpregnancy interval.
abstract: OBJECTIVE
  To assess the relationship between gestational lengths of the first and second pregnancies in the same women.
  
  
  METHODS
  Observational study.
  
  
  METHODS
  We used information from a dataset of over 500,000 pregnancies from 15 maternity units in the North West Thames, London. Data on the gestational length in days of the first pregnancy and the gestational length in days of the second pregnancy were correlated using regression models. First and second pregnancies were ascribed to the same women by identical maternal date of birth, ethnicity and maternal height (to within ±3cm).
  
  
  RESULTS
  There is a statistically significant cubic relationship between the gestational lengths of the first birth and the second birth (R 0.102, p<0.001). The gestational length of the second pregnancy is likely to be closer to 280 days than the first pregnancy. In the 20% of women who had an interpregnancy interval of less than one year, the next pregnancy was one day shorter for every three months less than 12.
  
  
  CONCLUSIONS
  Although the gestation of second pregnancies exhibits regression towards the mean of 280 days, there is still a clinically important tendency for both preterm and postdates pregnancies to recur. Prediction of an estimated delivery date for second pregnancies should take into account both the length of the first pregnancy and the interpregnancy interval if it is less than 12 months.
authors.affiliation: Academic Department of Obstetrics and Gynaecology, Imperial College London, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, United Kingdom. Electronic address: bonnie.ng@doctors.org.uk.
authors.forename: Ka Ying Bonnie
authors.initials: KY
authors.lastname: Ng
authors.affiliation: Academic Department of Obstetrics and Gynaecology, Imperial College London, Chelsea and Westminster Hospital, 369 Fulham Road, London SW10 9NH, United Kingdom.
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Steer
mesh_terms: D000328:Adult; D001721:Birth Intervals; D005260:Female; D006801:Humans; D007206:Individuality; D011247:Pregnancy / Q000706:statistics & numerical data*; D012044:Regression Analysis; D013997:Time Factors; D055815:Young Adult
keywords: Estimated delivery date; Interpregnancy interval; Post-dates; Pregnancy length; Preterm; Repeat gestational length