Doctors often advise women who are considered overweight by medical standards to lose weight before they try to have a baby—even though there's a lack of evidence backing this advice.

A randomized trial published in PLOS Medicine found that women with clinically defined obesity and unexplained infertility who lost weight before starting fertility treatments did not have a greater chance of conceiving or improved birth outcomes compared to those who didn’t lose weight.

In scientific literature, obesity has been correlated with difficulty conceiving, pregnancy complications and loss, among other conditions that can affect your health (and the baby’s). But it’s important to note that correlation doesn’t equal causation: Some studies say shedding pounds isn’t necessary; restricting calories can lead to pregnancy loss, while bariatric surgery has been linked to increased preterm deliveries or having a smaller baby.

Related: An OB/GYN answers the 10 top questions women want to ask about fertility—but don’t

For the purposes of this article, we’ve used the terms “overweight” and “obese” in a medical context. The World Health Organization deems anyone with a body mass index (BMI) of 25 kg/m2 and over as “overweight,” and a BMI over 30 is referred to as “obese.”

It’s also worth calling out the fact that BMI is a flawed metric: it doesn’t offer an accurate picture of health, as it doesn’t take into account the biological or genetic differences that need to be accounted for when it comes to the distribution of muscle and fat and corresponding disease risk. That’s just one of the reasons why recommending weight loss before starting fertility treatments may not be helpful if relying on BMI—a higher BMI doesn't necessarily equate to poor health.

Evaluating weight loss and fertility

In the study, Richard Legro, MD, professor and chair of the Department of Obstetrics and Gynecology at Penn State Health Milton S. Hershey Medical Center, looked at more than 300 women with obesity and unexplained infertility. The team wanted to see if losing weight before trying to conceive via fertility treatments would boost their chances of having a healthy baby.

All of the women had a BMI over 30 kg/m2. They regularly ovulated but had at least one year of unexplained infertility. The team didn’t include women with polycystic ovary syndrome (PCOS) or those who did not release eggs during their menstrual cycles.

They organized the women into two groups: Over the course of 16 weeks, the first group followed a diet plan, took weight loss medication and exercised; the second group simply increased their exercise. After that, the women began three cycles of fertility therapy consisting of ovarian stimulation and intrauterine insemination (IUI).

Related: IVF, IUI, ICI, and IVI: There are so many ways to make a family

In the end, the scientists didn’t find significant differences in the amount of pregnancies and healthy births between the two groups. Those in the first group (guided weight loss) lost 7% of their body weight; women in the second group did not lose any significant amount.

The authors say there were no improved live birth or healthy live birth rates in women who were considered obese or overweight and underwent a weight loss intervention before starting fertility treatments.

“We did not see increased conception rates with or without infertility treatment among women who lost weight,” Dr. Legro tells Motherly. “There is not strong evidence to recommend weight loss prior to conception in women who are obese with unexplained infertility,” the report states. In a statement, Legro acknowledges that advice differs from current clinical standards of care. 

Study limitations

Barry Witt, MD, medical director of WINFertility and a reproductive endocrinologist and OB/GYN from Connecticut who was not affiliated with the research, tells Motherly that the conclusion that weight loss is not beneficial can’t be generalized because the patients in the study had unexplained fertility. The research didn’t include patients with ovulation dysfunction. Also, the fertility treatment used—intrauterine insemination (IUI) with clomiphene—has a relatively low success rate, Dr. Witt notes. That can’t be compared with in vitro fertilization (IVF), where success rates are two to three times higher, he says.

“The weight loss was 7% on average, so it is possible that a greater amount of weight loss may be required to see a significant benefit,” Dr. Witt adds.

Is weight loss beneficial for fertility? 

Losing weight before conceiving can have other benefits, though, researchers say. For instance, some of the women who lost weight also lowered their blood pressure and decreased their waist circumference—two markers associated with long-term health and prevention of chronic disease.

So should you try to lose weight before TTC? Talk to your doctor, advises Nithya Gopal, MD, an OB/GYN in New York who was not on the study team. 

“Not all women need to lose weight prior to conceiving—the number on the scale is not everything,” Dr. Gopal says. 

“Rather, one should meet with their doctor to check on factors like blood pressure, overall cardiovascular health and a well-rounded diet prior to conception, whether naturally or via fertility treatments,” she advises. Health is made up of many different factors—not just weight. 

That said, Dr. Gopal finds in her experience that patients who achieve what’s deemed a normal BMI (the U.S. Centers for Disease Control and Prevention says this is when your BMI is between 18.5 to 24.9) before conception have lower rates of diabetes, high blood pressure, superimposed preeclampsia and large fetuses that can contribute to labor complications.

“These are major factors that play a pivotal role in the high- or low-risk nature of the pregnancy, which ultimately have a huge bearing on the health of the placenta and the newborn,” Dr. Gopal tells Motherly.

Eliminating weight stigma

Why is this study important? Because it helps start a conversation about eliminating weight stigma in a clinical setting.

Weight stigma is a common, well-documented issue in the medical world. It can include weight bias, such as having poor-fitting medical gowns or blood pressure cuffs that are only tailored for thinner individuals. Some doctors may view overweight or obese patients differently (perhaps as lazy or annoying), when in reality, weight issues may be outside of one’s control—weight is often dependent on genetics and certain socio-economic factors.

Related: 'Don't Weigh Me' cards help empower patients at the doctor's office

On the flip side, some patients may avoid seeking health care due to having a negative, weight-related experience in the past. That gets complicated during pregnancy, when they need care. 

Anna Bohnengel, RD, a fertility nutritionist from Oregon, advises women to focus on parameters of health other than weight loss, such as blood sugar balance, high-antioxidant diets, environmental endocrine disruptor avoidance and stress management.

The most common barrier to fertility Bohnengel sees is that women aren’t eating enough—especially of nutrient-dense foods.

Related: How to support a loved one going through infertility

“Focusing on weight loss is often not helpful because when this is the emphasis, women run the risk of under-eating and under-nourishing,” she says. This can put the body into a state of stress, alerting that it’s not safe to reproduce. She also notes the known harms of obesity on a pregnancy.

“I recommend that a woman focus on health markers and a healthy lifestyle, rather than the number on the scale. This goes no matter what route she's taking to motherhood,” Bohnengel says.

Sources

Forsum E, et al. Weight loss before conception: A systematic literature review. Food & nutrition research. 2013. doi:10.3402/fnr.v57i0.20522

Legro R, et al. Effects of preconception lifestyle intervention in infertile women with obesity: The FIT-PLESE randomized controlled trial. PLOS One. 2022. doi:10.1371/journal.pmed.1003883

Sakers, A, et al. Adipose-tissue plasticity in health and disease. Cell Press. 2022. Doi: 10.1016/j.cell.2021.12.016

Silvestris E, et al. Obesity as disruptor of the female fertility. Reproductive biology and endocrinology: RB&E. 2018. doi:10.1186/s12958-018-0336-z

Wise, L, et al. An internet-based prospective study of body size and time-to-pregnancy. Human Reproduction. 2010. DOI: 10.1093/humrep/dep360

Featured experts

Anna Bohnengel, MS, RD, a fertility nutritionist based in Oregon.
Nithya Gopal, MD, an OB/GYN based in New York.
Richard Legro, MD, professor and chair of the Department of Obstetrics and Gynecology at Penn State Health Milton S. Hershey Medical Center.
Barry Witt, MD, medical director of WINFertility and a reproductive endocrinologist and OB/GYN based in Connecticut.