Waiting time to pregnancy according to maternal birthweight and prepregnancy BMI.

BACKGROUND An association between a woman's own birthweight and her fecundity has been suggested, but no empirical data have been published on the association between maternal birthweight and waiting time to pregnancy (TTP). METHODS In the Danish National Birth Cohort (1996-2002), which is an ongoing study of 92 274 women and their pregnancies, information about TTP and prepregnancy BMI was collected during pregnancy. At the 7-year follow-up of the children, 21 786 mothers reported their own birthweight and whether they were born at term or preterm. The association between maternal birthweight and TTP is presented as adjusted odds ratios with 95% confidence intervals. RESULTS Low maternal birthweight (< or =2500 g for term and < or =1500 g for preterm birth) was associated with an increased risk of TTP of >1 year [term: 1.2 (1.0-1.5); preterm: 1.8 (1.1-3.1)]. The latter association was strongest in women with a BMI < 25 kg/m(2) [2.6 (1.4-4.7)]. High maternal birthweight (>4500 g for term and >3500 g for preterm) was also associated with an increased risk of TTP of >1 year [1.5 (1.0-2.0) and 1.3 (0.7-2.4), respectively], especially in women with a BMI > or = 25 kg/m(2) [1.8 (1.1-3.1) and 2.5 (1.0-6.4), respectively]. CONCLUSIONS High or low maternal birthweight was associated with TTP > 1 year. Longer waiting times in women with very low birthweight may reflect an effect of being born very preterm. Subfecundity may partly be programmed in foetal life by factors that cause or correlate with foetal growth.