Having a high body mass index (BMI) during pregnancy can have a major impact on your health and your baby's health. Find out about the possible complications, recommendations for weight gain and what you can do to promote a healthy pregnancy.

Obesity is diagnosed when a person's BMI is 30 or higher. To determine your BMI, divide your weight in pounds by your height in inches squared and multiply by 703. Or divide your weight in kilograms by your height in meters squared.

Having a high BMI can harm fertility by inhibiting regular ovulation. Even in women who regularly ovulate, the higher the BMI, the longer it may take to become pregnant. Some research also suggests that a higher BMI is associated with an increased risk of unsuccessful in vitro fertilization (IVF).

Having a high BMI during pregnancy increases the risk of various pregnancy complications, including:

• Miscarriage, stillbirth and recurrent miscarriage
• Gestational diabetes
• A pregnancy complication characterized by high blood pressure and signs of damage to another organ system, most often the liver and kidneys (preeclampsia)
• Heart problems
• Sleep apnea
• The need for a C-section and the risk of C-section complications, such as wound infections

Having a high BMI during pregnancy has been linked to an increased risk of various health problems for a baby, including:

• Congenital disorders
• Being significantly larger than average at birth (fetal macrosomia)
• Growth problems
• Childhood asthma
• Childhood obesity
• Cognitive problems and developmental delay

However, other factors also might play a role in these outcomes.

It's important to consider your pre-pregnancy weight and BMI when determining how much weight you need to gain during pregnancy. Work with your health care provider to find out what's best for you and to manage your weight throughout pregnancy.

Start by considering these guidelines for pregnancy weight gain and obesity:

• Single pregnancy. If you have a BMI of 30 or higher and are carrying one baby, the recommended weight gain is 11 to 20 pounds (about 5 to 9 kilograms).
• Multiple pregnancy. If you have a BMI of 30 or higher and are carrying twins or multiples, the recommended weight gain is 25 to 42 pounds (about 11 to 19 kilograms).

Rather than recommending that you gain a specific amount of weight, your health care provider might encourage you to focus on avoiding excessive weight gain during pregnancy.

If you have a BMI of 30 or higher, your health care provider will closely monitor your pregnancy. Your provider might recommend:

• Early testing for gestational diabetes. For women at average risk of gestational diabetes, a screening test called the glucose challenge test is often done between weeks 24 and 28 of pregnancy. If you have a BMI of 30 or higher, your health care provider might recommend the screening test at your first prenatal visit. If your test results show that your glucose levels are within the standard range, you'll likely repeat the screening test between weeks 24 and 28 of pregnancy. If the results show that your blood glucose levels are high, you'll need further testing. Your health care provider can advise you on blood sugar monitoring and control.
• Changes to your fetal ultrasound. A standard fetal ultrasound is typically done between weeks 18 and 20 of pregnancy to evaluate a baby's anatomy. But ultrasound waves don't easily penetrate abdominal fat tissue. This can interfere with the effectiveness of fetal ultrasound. Talk to your health care provider about the best approach for getting an accurate ultrasound.
• Screening for obstructive sleep apnea. Sleep apnea is a potentially serious sleep disorder that causes breathing to repeatedly stop and start during sleep. Women who have obstructive sleep apnea during pregnancy are at increased risk of preeclampsia and other complications. You'll likely be screened at your first prenatal visit. If obstructive sleep apnea is suspected, your health care provider may refer you to a sleep medicine specialist for evaluation and possible treatment.

You can limit the impact of having a high BMI on your health and your baby's health. For example:

• Schedule a preconception appointment. 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.
• Seek regular prenatal care. Prenatal visits can help your health care provider monitor your health and your baby's health. Tell your provider about any medical conditions you have — such as diabetes, high blood pressure or sleep apnea — and discuss what you can do to manage them.
• Eat a healthy diet. Work with your health care provider or a registered dietitian to maintain a healthy diet and avoid excessive weight gain. Keep in mind that during pregnancy, you'll need more folic acid, protein, calcium, iron and other essential nutrients.
• Be physically active. Consult your health care provider about safe ways to stay physically active during your pregnancy, such as walking, swimming or doing low-impact aerobics.
• Avoid risky substances. If you smoke, ask your health care provider to help you quit. Alcohol and illicit drugs are off-limits, too. Get your health care provider's OK before you start or stop taking any medications or supplements.

Having a BMI of 30 or higher during pregnancy can increase the risk of complications for you and your baby. Working with your health care provider can help you manage your risks and promote a healthy pregnancy.