Why Your Weight May Matter When You're Trying to Conceive

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Your weight matters when it comes to your fertility. Being overweight or underweight can make it harder to get pregnant. That's because excess or lack of fat can cause an imbalance in the reproductive hormones that make it possible for you to conceive during any given menstrual cycle.

According to the American Society of Reproductive Medicine (ASRM), more than 70% of women with weight-related infertility could get pregnant without fertility treatments if they brought their weight to a healthier level. You may be able to do this by simply adjusting your diet and activity levels. But you may need a doctor or specialist to help you uncover and treat underlying health issues that impact your weight.

How Obesity Affects Female Fertility

Obesity is a common cause of fertility problems. When people affected by obesity have trouble getting pregnant, it's often because they are not regularly ovulating. If you don't ovulate, you can't get pregnant.

Your ovaries naturally make the hormone estrogen, but fat cells make estrogen, too. If you have an excess of fat cells, your body will make excess estrogen. Just like when you take birth control or are pregnant (two states that include extra estrogen), you might stop ovulating as a result.

Fertility treatments can make it easier to get pregnant, but research shows that obesity can lower your odds of conceiving even with assisted reproductive technologies like in vitro fertilization (IVF). For that reason, it's a good idea to talk with a doctor about ways to achieve a weight that best suits your body, health, and pregnancy goals as early as possible.

How Being Underweight Impacts Female Fertility

In developed countries like the U.S., carrying too little weight is less common than carrying too much, affecting 5% of people versus 50% to 70% of people. While being underweight tends to cause fewer fertility issues than being overweight or obese, it can disrupt your ability to get pregnant.

People who are underweight or extremely lean have fewer fat cells. In response, their bodies make less estrogen than others. And just as ovulation is hampered by too much estrogen, it's also hindered by too little estrogen.

This is why some athletes in intensive training as well as people with eating disorders like anorexia may stop menstruating altogether. Dropping to 10% to 15% under normal body weight can cause you to stop having a period. Even if you are menstruating, your cycles may become irregular with extreme weight loss, which indicates you may not be ovulating regularly.

Fertility treatments, like medications that stimulate ovulation, are often successful for people who are underweight. And donated eggs are as likely to lead to a healthy pregnancy in underweight people as "normal"-weight people. Still, since there are health risks in babies born to people with low body weight, experts strongly suggest that you get a doctor's help to gain a healthy amount of weight before trying to conceive.

Other Weight-Related Female Fertility Issues

Just because a relationship between weight and fertility has been found, it doesn’t mean a doctor should ignore further testing if you happen to be over or underweight and are having difficulty conceiving. Sometimes, a hormonal imbalance leads to a weight problem, rather than the other way around. In these cases, treating the cause of that hormonal imbalance may make it easier to control your weight and boost your fertility.

Polycystic Ovary Syndrome (PCOS)

Polycystic ovary syndrome (PCOS) is a common cause of ovulation dysfunction. Up to 95% of people who seek out fertility treatments because they don't ovulate regularly have PCOS. Many people with PCOS make too much insulin, a hormone that converts blood sugar to energy. Excess insulin is linked to both obesity and irregular periods.

For people with PCOS who are overweight or obese, losing just 5% to 10% of body weight may help restore regular ovulation. Other people benefit from medication that treats their insulin issues directly. The insulin-resistance drug metformin is proven to help many people with PCOS lose weight as well as ovulate more regularly.

Thyroid Disorders

Thyroid disorders are also known to cause problems with both weight and fertility. When you have an underactive thyroid, or hypothyroidism, you don't make enough of two hormones, known as T3 and T4. These hormones play a role in metabolism—which is linked to weight control—as well as ovulation.

Many people with hypothyroidism can take medications to help their bodies more effectively release an egg each menstrual cycle. In one study of some 400 people with infertility, nearly one-quarter of participants were found to have hypothyroidism, and 76% of those people were able to get pregnant within one year of treatment.

Weight and Male Fertility

While the gestational parent naturally may focus on improving their own health when trying to get pregnant, both partners must consider how their weight may be affecting their odds of conceiving. In 45% to 50% of cases when a couple can't get pregnant, male fertility is a factor, and mounting research points to weight issues as a major contributor.

The rise in obesity rates across the developed world has nearly paralleled the increase in male infertility. Excess weight can throw off the healthy balance of male hormones similar to the way it disrupts female hormones. Male obesity is linked to poor sperm quality and motility—the ability of sperm to move easily and swiftly in the female reproductive tract. A study looking at BMI and waist circumference found that ejaculate volume declines as BMI and waist circumference goes up.

Meanwhile, a systematic review of studies found that low BMI is associated with poor semen quality, too. Researchers are still working to find out if that's mainly because low levels of body fat negatively impact male hormones or whether unhealthy lifestyle factors sometimes linked to weight loss may harm sperm in other ways.

Body Mass Index (BMI) is a dated, biased measure that doesn’t account for several factors, such as body composition, ethnicity, race, gender, and age.

Despite being a flawed measure, BMI is widely used today in the medical community because it is an inexpensive and quick method for analyzing potential health status and outcomes.

A Word From Verywell

Many people who are underweight, overweight, or obese have no trouble getting pregnant. However, if you have weight issues and want to get pregnant very soon, or have been trying to conceive for six months or more without success, it's worth checking in with a doctor.

A doctor can do a complete physical and advise you about whether losing or gaining weight may help your chance of conceiving. They can also do certain tests to see if conditions like PCOS or thyroid issues may be affecting your weight and odds of conceiving.

One thing to avoid at all costs is an extreme diet. There are many "fertility diet" plans online; don't go on one without talking to a doctor first. Losing weight slowly, while nourishing your body in satisfying ways, is best for your fertility and your overall health. Certified nutritionists or registered dietitians are the best people to guide you.

If you're significantly overweight or underweight, don't feel like your situation is hopeless. Losing or gaining even small amounts of weight when you're outside a healthy zone can be helpful for reproductive, physical, and mental wellness.

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11 Sources
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  1. U.S. Department of Health and Human Services Office on Women's Health. Weight, fertility, and pregnancy.

  2. Bates GW. Abnormal body weight: A preventable cause of infertility. American Society for Reproductive Medicine.

  3. American Society for Reproductive Medicine. Obesity and reproduction: A committee opinion. Fertil Steril. 2021 Nov;116(5):1266-1285. doi:10.1016/j.fertnstert.2021.08.018

  4. Bellver J. In vitro fertilization in underweight women: Focus on obstetric outcomeFertility and Sterility. 2020;113(2):323-324. doi:10.1016/j.fertnstert.2019.10.009

  5. Dennett CC, Simon J. The role of polycystic ovary syndrome in reproductive and metabolic health: Overview and approaches for treatment: Table 1Diabetes Spectr. 2015;28(2):116-120. doi:10.2337/diaspect.28.2.116

  6. Johnson NP. Metformin use in women with polycystic ovary syndrome. Ann Transl Med. 2014;2(6):56. doi:10.3978/j.issn.2305-5839.2014.04.15

  7. American Society for Reproductive Medicine. Hypothyroidism and pregnancy: What should I know?.

  8. Verma I, Juneja S, Sood R, Kaur S. Prevalence of hypothyroidism in infertile women and evaluation of response of treatment for hypothyroidism on infertilityInt J App Basic Med Res. 2012;2(1):17. doi:10.4103/2229-516X.96795

  9. Craig JR, Jenkins TG, Carrell DT, Hotaling JM. Obesity, male infertility, and the sperm epigenomeFertility and Sterility. 2017;107(4):848-859. doi:10.1016/j.fertnstert.2017.02.115

  10. Eisenberg ML, Kim S, Chen Z, Sundaram R, Schisterman EF, Buck louis GM. The relationship between male BMI and waist circumference on semen quality: Data from the LIFE study. Hum Reprod. 2014;29(2):193-200. doi:10.1093/humrep/det428

  11. Guo D, Xu M, Zhou Q, Wu C, Ju R, Dai J. Is low body mass index a risk factor for semen quality? A PRISMA-compliant meta-analysisMedicine. 2019;98(32):e16677. doi:10.1097/MD.0000000000016677