Getting pregnant and having a healthy pregnancy is possible at a variety of weights. However, being underweight or overweight can increase risks for both mom and baby. Understanding the impact of weight on fertility and pregnancy outcomes can help women reach a healthy weight before conceiving.
What is the BMI classification for underweight?
Body mass index (BMI) is a measure of body fat based on height and weight. It is used to classify weight status:
- Underweight = BMI less than 18.5
- Normal weight = BMI 18.5–24.9
- Overweight = BMI 25–29.9
- Obese = BMI 30 or greater
A BMI below 18.5 is considered underweight. However, BMI has some limitations and does not directly measure body fat. A healthcare provider will also consider overall health, body composition, and any eating disorders when assessing if a woman is underweight.
How does being underweight affect fertility?
Being underweight can cause hormone imbalances that disrupt ovulation and make it harder to get pregnant. Specific effects include:
- Irregular or absent menstrual periods
- Low estrogen levels
- Failure to release an egg from the ovaries (ovulate)
- Decreased sexual desire
Underweight women may need to gain weight to restore ovulation and improve their chances of conception. Even gaining 5-10 pounds can kickstart ovulation for some underweight women.
What BMI is too low for pregnancy?
There is no definitive BMI cut-off that is too low for pregnancy. However, research suggests that fertility and pregnancy risks increase with lower BMIs, especially under 18.5:
- BMI 16.0 – 17.4: 12 times more likely to have irregular periods
- BMI 15.0 – 15.9: Over 4 times more likely to have ovulation problems
- BMI less than 15: May completely stop ovulation
The risks are highest for women with BMIs under 15. But any BMI under 18.5 can impact fertility, and women may need to gain some weight to improve ovulation.
What are the risks of pregnancy at a low weight?
In addition to reduced fertility, being underweight during pregnancy raises risks for complications and poor outcomes:
- Miscarriage – Underweight women have double the risk of first trimester miscarriage.
- Preterm birth – Increased risk of delivering 3-4 weeks early.
- Low birth weight – Raises the chance of having a small baby.
- Stillbirth – Risk is 2-3 times higher than normal weight women.
- C-section – Underweight women have a 60% higher C-section rate.
- Preeclampsia – Dangerous high blood pressure in pregnancy is more common.
- Gestational diabetes – Being underweight does not protect against developing diabetes during pregnancy.
Babies born to underweight moms also face risks like birth defects, low blood sugar, and jaundice.
Why is low weight dangerous during pregnancy?
There are several reasons why low BMI elevates risks:
- Less blood volume and smaller uterus reduces oxygen and nutrients to the baby.
- Low fat stores limit energy reserves needed for fetal growth.
- Nutritional deficiencies are common.
- Hormonal imbalances can disrupt placental development.
- Underweight stresses the body, increasing inflammation and oxidative stress.
Together, these effects impair the growth and development of the fetus and raise risks for complications.
What is the healthiest minimum weight for pregnancy?
Rather than a specific minimum weight, experts recommend reaching a normal BMI of 18.5-24.9 before pregnancy. This provides energy reserves for mom and baby without elevating risks:
- BMI 18.5-24.9 has the lowest risks of infertility and complications.
- Reaching a normal BMI can restore ovulation and improve fertility.
- Normal weight women have the best outcomes for both mom and baby.
However, pregnancy is still possible for underweight women after addressing any nutritional deficiencies and health conditions. Monitoring fetal growth and maternal health is important if BMI is very low.
What should underweight women do before pregnancy?
Tips for underweight women planning a pregnancy:
- Gain weight gradually with a healthy diet until BMI reaches normal range.
- Take a prenatal vitamin with folic acid at least 3 months before conception.
- See your healthcare provider for any needed health screenings and management of chronic conditions.
- Discuss optimal weight gain goals for pregnancy with your provider.
- Work with a nutritionist to design a balanced diet and meal plan.
Making healthy lifestyle changes and gaining some weight can help get your body ready for the demands of pregnancy and lower risks.
What is the ideal weight gain in pregnancy for underweight women?
The Institute of Medicine provides this guidance on weight gain during pregnancy based on pre-pregnancy BMI:
| Pre-Pregnancy BMI | Recommended Weight Gain |
|---|---|
| Underweight (less than 18.5) | 28-40 pounds |
| Normal weight (18.5-24.9) | 25-35 pounds |
| Overweight (25.0-29.9) | 15-25 pounds |
| Obese (30.0 and above) | 11-20 pounds |
Underweight women are encouraged to gain more than normal BMI women to provide adequate energy stores for fetal growth and development.
What causes low BMI?
Some reasons women may be underweight include:
- Genetic predisposition to being slender
- Hyperthyroidism or overactive metabolism
- Eating disorders like anorexia or bulimia
- Inflammatory bowel diseases
- Celiac disease
- Diabetes
- Cancer or chronic infections
- Psychiatric conditions like depression or anxiety
- Smoking and substance abuse
Ruling out any underlying medical conditions is important. Working with a multidisciplinary team can help underweight women gain weight safely before conception.
What food and diet tips help underweight women gain weight?
A nutritionist can provide personalized advice, but general diet tips include:
- Eat frequently – have 6 small meals instead of 3 large ones.
- Choose higher calorie foods like avocados, nuts, dried fruit, full fat dairy.
- Add extra oils, butter, protein powders to meals.
- Drink milkshakes or smoothies between meals.
- Keep portion sizes moderate to avoid feeling too full.
- Exercise regularly but don’t overdo high intensity workouts.
- Get adequate rest and sleep.
Gaining 1-2 pounds per week allows for gradual weight gain without adding too much fat mass.
What is the minimum weight to get pregnant with IVF?
There are no definitive BMI cutoffs for IVF, but very low weight can lower success rates. Some IVF centers recommendations include:
- BMI above 17-18 for optimal IVF outcomes.
- BMI 18-19 may require higher medication doses.
- BMI below 17 generally recommends weight gain before treatment.
Talk with your fertility specialist about your weight status and options to maximize IVF success. Underweight women may need to gain weight first or use a gestational carrier.
Conclusion
While women can conceive and have healthy pregnancies at a wide range of weights, low BMI under 18.5 raises risks. Reaching a normal BMI of 18.5-24.9 before pregnancy provides the healthiest outcomes for both mother and baby. Underweight women planning a pregnancy should focus on gradual weight gain, nutrition, and optimizing health with their healthcare team.