Objective

To assess the relationship between gestational lengths of the first and second pregnancies in the same women.

Study design

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 ±3 cm).

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.