Overweight women are more than twice as likely to miscarry an IVF baby compared with those whose weight is healthier, fertility doctors say. The increased risk is so great they believe a warning should be included in counselling for couples before they embark on a course of fertility treatment.
Women who conceive naturally are known to have a greater chance of miscarrying if their body mass index (BMI) is 25 or higher, but the picture has been less clear for women carrying babies produced by in-vitro fertilisation (IVF), or another technique called intra-cytoplasmic sperm injection (ICSI).
"Overweight women wishing to get pregnant by spontaneous conception are already counselled to lose weight before trying for a baby," said Tarek El-Toukhy, a fertility specialist who led the study at the assisted conception unit of Guy's and St Thomas' Hospital in London.
"Our findings have shown clearly that women undertaking ART [assisted reproductive technology] should be strongly encouraged to heed this advice in order that they can have the best possible chance of obtaining and maintaining a pregnancy," he added.
Overweight mothers have a higher risk of developing other medical conditions that can threaten their pregnancy, including high blood pressure, pre-eclampsia, diabetes, premature delivery and post-partum bleeding.
El-Toukhy's team examined the medical records of 318 women who each had one embryo implanted during fertility treatment at the clinic between January 2006 and December 2009. The women were divided into two groups: 185 had a healthy BMI between 18.5 and 24.9, while 133 had a BMI of 25 or above. Of the latter group, 19 were obese, defined as having a BMI of 30 or more.
The study, reported today at a meeting of the European Society of Human Reproduction and Embryology in Rome, found a miscarriage rate of 22% among women with a healthy BMI, compared with a 33% miscarriage rate for the overweight women.
After adjusting their data to take account of the women's age, history of infertility and miscarriage, and lifestyle factors such as smoking, the researchers concluded that being overweight more than doubled the miscarriage rate.
"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," said Vivian Rittenberg, a fertility doctor who took part in the study.
Rittenberg said many studies that have examined the issue in the past have been hard to interpret, not least because doctors looked at miscarriage rates after implanting several embryos at once at different stages of development.
"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," she said.
