Why some experts don’t recommend intermittent fasting if you’re trying to get pregnant

Forget about simply cutting processed foods or carbs from your diet: Celebs like Hugh Jackman and Kourtney Kardashian reportedly aren’t eating much—if at all—some days. Known as intermittent fasting (IF), the goal of the buzzy eating plan is to speed up metabolism, encourage healthy weight management, and improve overall health by following certain time-specific restrictions on eating. But if the idea of severely cutting calories or skipping meals on some days sounds iffy to you, there’s good reason to be hesitant—especially if you’re a woman.

In theory, periods of fasting allow your body to churn through the sugar in your blood, lowering your insulin levels so you eventually use stored fat as energy. Some research suggests that IF might improve blood sugar, pressure, and cholesterol levels, promote healthy weight management, and reduce Type 2 diabetes risk—although most studies so far are in animals.

The calculations on IF benefits, however, may change for women. “In my patients, I’ve seen a lot of females who can’t go above 18 hours of fasting, while the guys can do three-day water fasts,” says Ruvini Wijetilaka, MD, a board-certified internal medicine physician at Parsley Health in New York City. “I’m not saying all fasting is bad. But severe caloric restriction in IF is not beneficial fertility-wise.”

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As it’s presented in high school health class, pregnancy seems pretty straightforward: egg plus sperm equals baby. But conception involves a delicate balance of hormones and timing that can be affected by everything from your weight to what you eat (or what you’re not eating). For example, having a very high BMI can mess with a person’s hormone balance, leading to irregular ovulation, explains Aaron Styer, MD, a reproductive endocrinologist and a founding partner and co-medical director of the Colorado Center for Reproductive Medicine in Boston. People who are underweight can also struggle to get pregnant. “The female pituitary gland can sense calorie intake. Below a certain level, it can create a signal from the brain for ovulation to be reduced or absent,” says Dr. Styer.

Want to know more about IF? Check out this guide from a top registered dietitian:

Similarly, intermittent fasting might affect levels of the hormone leptin, explains Dr. Styer. Leptin is produced by the body’s fat cells and controls not only your metabolism but levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), the hormones that regulate ovulation. Too little leptin, and there might be issues with fertility down the road. “It could possibly sub-optimize the ability to conceive. But it’s not confirmed,” he says. Dr. Wijetilaka adds that one small study in mice showed that very severe caloric restriction brought on by some forms of IF shrunk ovary size and decreased LH and FSH levels. But this was a study in mice and thus isn’t a certainty in humans.

If you do conceive, IF may not foster the ideal conditions for a fast-growing baby, either. “Pregnancy requires lots of resources. You don’t want to cause undue stress from restricting calories,” says Dr. Styer. People who are pregnant actually need to be eating more calories per day—340 extra calories per day in the second trimester, and around 450 extra per day during the third trimester. Trying IF, or sticking with it, during pregnancy should thus be put on hold until you can talk to your doctor about it and see what they think.

It’s important to note that this is mostly a theory—there’s not a ton of research out there on humans that has looked into IF’s potential effect on fertility. There are also many different ways to do IF, some of them with more potential risks than others. Both Dr. Wijetilaka and Dr. Styer suggest steering clear of caloric restriction types of IF like 5:2 (eating 500 calories per day a couple of times per week), alternate-day fasting (capping calories at 500 every other day), and 24-hour fasts (not eating at all for one or two days per week) if you’re trying to get pregnant. However, time-restricted eating plans like 16:8, where you eat whatever you want during a set eight-hour period during the day and fast for the remaining 16 hours, may be safer for fertility, Dr. Wijetilaka says. She adds that while fasts over 16 hours are risky, she generally recommends a 12-hour fast to give the body and metabolism a rest.

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Dr. Styer agrees that time-restricted eating is safer, but he still doesn’t recommend IF for women who are trying to get pregnant. “The female pituitary gland can be sensitive to caloric intake and diet in general,” he says. But don’t sweat it if you’ve already given IF a go in the past or if you want to try it out and aren’t planning on conceiving in the near future. “Your body is resilient. I don’t see risks for a future pregnancy,” Dr. Styer adds.

As with all eating plans, it’s a good idea to talk to your doctor or dietitian about intermittent fasting before diving in, especially if you’re thinking about pregnancy in the near future. While maintaining a healthy BMI is important for fertility and pregnancy, Dr. Styer says, IF might not be the way to get there. “Intermittent fasting is intriguing, and many people have lost weight successfully. But studies are limited in fertility and reproduction,” he says.

Yes, breakfast is the most important meal of the day—so where does that leave people who are intermittent fasting? And can you even drink coffee on IF?