Obese women undergoing IVF may be half as likely as their normal-weight counterparts to have a baby.
Researchers at Brigham and Women's Hospital in Boston found that of 1700 women who underwent IVF at their centre, those who were obese were up to 50% less likely to become pregnant or give birth.
Obese women also generally had lower oestrogen levels and produced fewer normally fertilized embryos - pointing to potential reasons that obese women are less likely to have a baby via IVF.
It's known that obesity can affect a woman's ability to conceive naturally.
However, "there is still a fair amount of uncertainty and debate as to how obesity affects IVF outcome," said Dr. Divya K. Shah, the lead researcher on the new study.
What is IVF?
IVF involves fertilizing a woman's eggs
- either the patient's own or eggs from a donor - in a lab dish, then implanting any resulting embryos in the patient's uterus a few days later.
The current findings, Shah said, suggest that "the eggs of obese women do not fertilize as well as those of normal-weight women."
Whatever the underlying reason, Shah said the "take-home message" is that women should try to get down to a healthy weight before they start IVF.
For their study, which appears in the journal Obstetrics & Gynaecology, Shah's team reviewed records from 1,721 women who underwent one round of IVF treatment at their infertility centre between 2007 and 2010. All of the women used their own eggs.
Overall, the researchers found, obese women were one-third to 50% less likely than normal-weight women to become pregnant - even after factors like age and the cause of the infertility (if known) were taken into account.
Of 1,023 normal-weight women whose average age was about 36, 440 got pregnant after one IVF attempt and 348 had live births. The odds of pregnancy and live birth were 50% lower among women who were extremely obese - having a body mass index (BMI) of 40 or higher.
Women who were moderately obese also had a lesser chance of having a baby than normal-weight women did, but that difference did not reach statistical significance - which means it could be a chance finding.
"There is evidence from other studies," Shah said, "that weight loss increases the chances of becoming pregnant without infertility treatment, and decreases the risk of miscarriage and other pregnancy complications."
Achieving a healthy weight might be wise not only for obese women, but underweight women as well.
Shah's team found that women with a low BMI were less likely to become pregnant or have a baby than normal-weight women were -- though, again, that difference was not quite statistically significant.
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