Extra weight can make it hard for you to get pregnant. For example, polycystic ovary syndrome, or PCOS, is one of the most common reasons for infertility in women and can also cause obesity. Overweight and obesity affect fertility by:

• Preventing ovulation. Your ovaries make the female hormone estrogen. Fat cells also make estrogen. As you gain weight, your fat cells grow and release more estrogen. Too much natural estrogen can cause your body to react as if you are taking hormonal birth control with estrogen (like the pill, shot, or vaginal ring) or are already pregnant. This can prevent you from ovulating and having a monthly period.
• Preventing fertility treatments from working. Obesity may lower your chances of getting pregnant with certain fertility treatments, such as in vitro fertilization (IVF).

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. This is especially true if you are losing weight because you are not eating enough or because you are exercising too much, which may be signs of an eating disorder like anorexia nervosa.

In order to get pregnant, you need to ovulate or release an egg from the ovary so it can be fertilized by a man’s sperm. Then your body needs to be able to support a developing baby in the womb.

Yes. Every woman is different, but studies show that for women who have overweight or obesity, losing weight raised their chances of getting pregnant. Losing weight also helped menstrual cycles return to normal. Talk to your doctor or nurse about how to lose weight safely.

Women who need to gain weight before getting pregnant should gain weight gradually and talk to their doctor or nurse about how to gain weight safely.

How much weight you should gain during pregnancy depends on your body mass index (BMI) before getting pregnant.

If you have:

• Underweight (BMI of less than 18.5), you should gain 28 to 40 pounds
• Normal weight (BMI of 18.5 to 24.9), you should gain 25 to 35 pounds
• Overweight (BMI of 25 to 29.9), you should gain 15 to 25 pounds
• Obesity (BMI of 30 or greater), you should gain no more than 11 to 20 pounds

Talk to your doctor, nurse, or midwife about how much weight is safe to gain during pregnancy.

The second and third trimesters of pregnancy are associated with dramatic changes in maternal anatomy and physiology. The most obvious anatomical sign of pregnancy is the dramatic enlargement of the abdominal region, coupled with maternal weight gain. This weight results from the growing fetus as well as the enlarged uterus, amniotic fluid, and placenta. Additional breast tissue and dramatically increased blood volume also contribute to weight gain (Table). Surprisingly, fat storage accounts for only approximately 2.3 kg (5 lbs) in a normal pregnancy and serves as a reserve for the increased metabolic demand of breastfeeding.

During the first trimester, the mother does not need to consume additional calories to maintain a healthy pregnancy. However, a weight gain of approximately 0.45 kg (1 lb) per month is common. During the second and third trimesters, the mother's appetite increases, but it is only necessary for her to consume an additional 300 calories per day to support the growing fetus. Most women gain approximately 0.45 kg (1 lb) per week.

Having overweight or obesity during pregnancy raises your risk for problems during pregnancy. Also, even if you do not have overweight or obesity, gaining more weight than recommended can cause the same problems.

These include:

• Gestational hypertension (high blood pressure during pregnancy). If not controlled during pregnancy, gestational hypertension may lead to a more serious condition called preeclampsia.
• Gestational diabetes (diabetes that starts during pregnancy). Having overweight or obesity raises the risk for gestational diabetes. Women who have had gestational diabetes also have a higher lifetime risk for obesity and type 2 diabetes. Gestational diabetes can cause low blood sugar in the infant. Unborn babies may also be larger, which could injure the baby or the mother during birth.
• Increased risk for C-section

Talk to your doctor, nurse, or midwife about healthy weight gain during pregnancy to help lower your risk for these health problems.

Babies born to mothers with overweight or obesity are at higher risk for health problems, including:

• Neural tube defects, such as spina bifida
• Heart defects
• Low blood sugar and larger body size, if the mother has gestational diabetes
• Obesity, type 2 diabetes, and high cholesterol

Babies born to mothers who are underweight (women with BMIs lower than 18.5) are at higher risk for health problems, including:

• Premature birth (also called preterm birth), or childbirth before 37 weeks of pregnancy
• Low birth weight (smaller than 5 1/2 pounds). These infants are at risk for health and development problems as they get older.