Dr. Rittenberg and colleagues analysed all pregnancies arising after single blastocyst (five-day embryo) transfer (SBT) performed in their clinic over a four year period, between January 2006 and December 2009. The 318 women were divided into two groups according to their BMI at the start of their treatment cycle. One group of 185 women had BMI in the normal range, from 18.5-24.9, and the other group (133 women) was made up of those who had a BMI of 25 or above. Of this group, 19 (14%) were obese, defined as having a BMI of 30 or more.
Overall, 26% of women miscarried before 20 weeks gestation. The researchers found that the miscarriage rate was significantly lower in women with normal weight (22%) compared to women who were overweight (33%). After adjusting for other variables that might have skewed the results such as age, duration of infertility, smoking and a history of previous miscarriage, they were able to show that being overweight or obese more than doubled the risk of miscarriage.
"Although there is evidence that miscarriage rates are higher in overweight women who conceive spontaneously, there were conflicting views about the effect of increased weight on the outcome of pregnancies occurring after IVF and ICSI," Dr. Rittenberg said. "The difficulty of interpreting the studies to date is that they are heterogeneous, their thresholds for defining obesity vary, and they assess the outcome in relation to BMI following the transfer of multiple embryos at various stages of development. Our study differs in that we transferred only one embryo at a specific stage of development, and were therefore able to provide clear evidence of the deleterious effect of being overweight on the chances of miscarriage."
Being overweight can lead to a number of other pregnancy risks including high blood pressure, pre-eclampsia, diabetes, premature delivery and post-partum bleeding. Besides the inherent medical dangers of these conditions, they can lead to lengthy hospital stays for the pregnant woman. Additionally, babies of obese mothers are more likely to have a high birth weight and thus may face an increased rate of delivery by Caesarean section, which in itself can cause complications for both mother and baby.
"Overweight women wishing to get pregnant by spontaneous conception are already counselled to lose weight before trying for a baby," said Dr. Tarek El-Toukhy, Consultant in Reproductive Medicine at the Assisted Conception Unit, Guy’s and St. Thomas’ Hospital NHS Foundation Trust, London, UK, who led the study. "Our findings have shown clearly that women undertaking ART should be strongly encouraged to heed this advice in order that they can have the best possible chance of obtaining and maintaining a pregnancy."