An investigation of the relationship between obesity, twinning rates and perinatal outcomes in Aberdeen

Aim: To examine whether natural twins are more likely to be born to women who are overweight or obese than to women of normal body mass index (BMI). And to look at whether maternal BMI affects adverse outcomes among twins.

Methods: A database was created containing information on pregnancies and deliveries booked at Aberdeen Maternity Hospital. Twin deliveries were categorised as natural conceptions or those resulting from assisted reproductive technology (ART). The association between BMI and twinning was described for natural and ART pregnancies, taking into account maternal characteristics. The relationships between BMI and adverse perinatal outcomes were investigated using logistic regression for natural twins, ART twins and singleton deliveries.

Results: Between 1976 to 2006, there were a total of 1135 twin pregnancies amongst Aberdeen city residents. The twinning rate increased from 10 to over 15 twin pregnancies per 1000 maternities. While this trend appears to have been driven by ART, there was also a significant increase in naturally conceived twins. Women with natural twin pregnancies were on average older, taller, heavier and of higher parity than those with singleton pregnancies, but the risk of twin pregnancy did not vary significantly with BMI. Women with higher BMIs were more likely to have premature babies (<28 weeks) and this effect was stronger among twins than singletons. BMI was associated with caesarean delivery more strongly among singletons than among twins, but birth weight discordance was not affected by maternal BMI.

Conclusions: No relationship between BMI and twinning has been found in this population.