A prospective study of pregravid oral contraceptive use in relation to fetal growth

Objective Because oral contraceptives are so widely used, any health consequences may have substantial public health implications. Whether pregravid oral contraceptives could affect subsequent pregnancies has not been adequately studied. The study objectives were to examine whether pregravid oral contraceptive use affects fetal growth and pregnancy hormone levels. Design A prospective study of pregnant women followed through pregnancy. Setting A major teaching hospital in Boston, USA. Population Two hundred and sixty Caucasian pregnant women, with a mean age of 31, and a parity of no more than two. Seventy-nine percent of the women were pregravid oral contraceptive users. Methods Exposure and covariate data were collected through structured questionnaires. Blood was drawn for hormonal analysis during the 16th and 27th gestational week. Information on pregravid oral contraceptive use included duration and recency of use, and oral contraceptive formulation. Multivariate regression models were used to examine the effect of pregravid oral contraceptive use on birth outcomes and the studied pregnancy hormones. Main outcome measures Birthweight, placental weight, gestational age, pregnancy hormone levels of oestriol and progesterone at 16th and 27th gestational week. Results Adjusting for confounders, pregravid oral contraceptive use increased birthweight (mean difference +207.3 g, 95% CI = +77.6 to +337.1) and placental weight (mean difference = +64.9 g, 95% CI = +13.0 to+116.9) compared with never use. Women with prior oral contraceptive use had higher levels of serum progesterone (P = 0.002) and oestriol (P = 0.12) at the 27th gestational week measurement. The effect on birthweight, placental weight and hormones was stronger among those using oral contraceptives in the previous year and those using a high progestin/high oestrogen potency preparation. Conclusions Pregravid oral contraceptive use is positively associated with fetal growth, and this effect may be mediated through oestriol and progesterone.