Underweight and fertility Being underweight (BMI under 18.5) can reduce a woman's fertility by causing hormone imbalances that affect ovulation and the chance of getting pregnant. Compared to women in the healthy weight range, women who are underweight are more likely to take more than a year to get pregnant.
The normal range (a BMI between 18.5 and 24.9) is the sweet spot for getting pregnant, as far as body weight is concerned. A study from 2022 looked at fertility ranges based on BMI. The authors found that conception was affected by both overweight and underweight women, and fertility was highest at a BMI of 19.5.
In contrast, when BMI ≥19.5 kg/m2, each unit increase in BMI predicted a 3% increase in the risk of infertility.
The ideal BMI for getting pregnant is between 18.5 and 24.9.
If you are underweight (your BMI is 18.5 or less), you may have problems getting pregnant. Being underweight can cause your body to stop making estrogen. This can cause irregular menstrual cycles. You may stop ovulating and getting your period.
Having a BMI below 18.5 is classed as underweight, which can make it harder to get pregnant. Having a low BMI can also cause your periods to become irregular or stop. This can be a sign that you are not ovulating (releasing an egg from your ovary each month), which is needed to get pregnant.
Guidelines indicate that your chances of getting pregnant with IVF are better if your BMI is between 19 and 30. A BMI over 30 has a significant negative effect on IVF pregnancy success rates. IVF success rates are most favourable in patients with low and normal BMIs.
Being underweight (BMI under 18.5) can reduce a woman's fertility by causing hormone imbalances that affect ovulation and the chance of getting pregnant. Compared to women in the healthy weight range, women who are underweight are more likely to take more than a year to get pregnant.
If you have a BMI of 30 or higher and you're considering getting pregnant, talk to your health care provider. Your provider might recommend a daily prenatal vitamin and refer you to other health care providers — such as a registered dietitian — who can help you reach a healthy weight before you conceive.
Obesity during pregnancy can pose increased risk for a mother and her baby. A body mass index greater than 30 classifies a person as obese. If you are obese, your healthcare provider will usually increase the supervision of your pregnancy.
A BMI of 18.5 or lower may cause you to have a more difficult time getting pregnant and increases the risk of preterm birth. Many women with a BMI below 18.5 have irregular or absent periods, and may not be ovulating regularly. Gaining just a few pounds may make a difference in your likelihood of conceiving.
However, research shows that women with a raised BMI of 30kg/m2 or more (i.e. obese) at the start of pregnancy are at increased risk of complications during pregnancy and labour and the higher your BMI, the greater the risk of complications.
Many women who carry excess weight still ovulate, but it appears the quality of the eggs they produce is reduced. The evidence for this is that among women who ovulate, each unit of BMI above 29 reduces the chance of achieving a pregnancy within 12 months by about 4%.
If your BMI is over 30: Significantly Overweight
This indicates an unhealthy condition, your excess mass is putting you at risk for heart disease, diabetes, high blood pressure, gall bladder disease, circulation problems, and some cancers. You should lose weight by changing your diet and exercising more.
Generally speaking a “normal” female BMI is 18 - 25 but because being too light can impact a person's ability to conceive, the ideal range for hopeful parents is 20 - 25.
If your BMI is: under 18.5kg/m2 – you are considered underweight and possibly malnourished. 18.5 to 24.9kg/m2 – you are within a healthy weight range for young and middle-aged adults. 25.0 to 29.9kg/m2 – you are considered overweight.
The findings suggested that those classed as very underweight -- with a body mass index of less than 18.5 -- were much more likely to miscarry in the first trimester.
Researchers from Harvard Medical School have now shown another mechanism that may contribute to the lower success rate: Obese women are more likely to have abnormalities in their eggs that make them impossible to fertilize.
The World Health Organisation (WHO) defines a healthy adult BMI as being between 18.5 and 25. A BMI less than 18.5 indicates underweight (possibly caused by disease, malnutrition or eating disorders such as anorexia nervosa) and a BMI over 25 as overweight (possibly caused by poor diet choices or lack of exercise).
18 or lower: underweight. 18.5 to 24.9: normal, healthy weight. 25 to 29.9: overweight. 30 or higher: obese.
A body mass index of anything under 18.5 is considered underweight and can have potential health consequences, just like a BMI that's considered obesity. Speak with your doctor if you have a BMI of 16 because this is dangerously low.
For pregnant women your BMI calculation will be based on your weight before pregnancy. Your midwife may measure your height and weigh you to work out your BMI at your first antenatal visit (the booking appointment). This will help them find out if you need some extra care during your pregnancy.
The amount of weight you gain during pregnancy is important for the health of your pregnancy and for the long-term health of you and your baby. If you were obese (body mass index 30.0 and above) before pregnancy, you should gain 11-20 pounds during pregnancy.
The primary weight management strategies during pregnancy are dietary control, exercise, and behavior modification. The goal is to avoid excessive gestational weight gain, which is common in obese women.