What Does My Pregnant Belly Size Mean?

Starting around 20 weeks, your health care provider will begin to measure your pregnant belly size from your pubic bone to the top of your uterus (aka the fundus) as part of routine prenatal checkups. This measurement is called fundal height, and it becomes an important indicator of prenatal health about halfway through pregnancy. It can tell your health care provider whether your fetus is growing as expected based on where you are in your pregnancy.

Read on to learn what your pregnant belly size means, why it matters, and what to do if you measure bigger or smaller than expected.

What Does Pregnant Belly Size Tell You?

According to the American College of Obstetricians and Gynecologists (ACOG), fundal height measurement estimates how your fetus is growing and helps health care providers screen for fetal growth restriction. In fact, a measurement between 32 and 34 weeks can be highly accurate in detecting fetuses that are not growing as expected.

However, some factors, like obesity and uterine fibroids, can interfere with accurate fundal height measurements. In these cases, ultrasound may be a better measurement and screening tool.

How Is Pregnant Belly Size Measured?

There's a simple formula for calculating how large your pregnant belly is expected to be at any given point in your pregnancy. Simply stated, your fundal height should be the same size in centimeters as the length of your gestation, plus or minus a couple of centimeters.

So, start with the number of weeks you're pregnant, then add two to that number and also subtract two from that number. This simple math will give you the range your pregnant belly size should be within. For example, belly measurement at 30 weeks should be between 28 and 32 centimeters. If you're 25 weeks along, your belly should measure between 23 and 27 centimeters.

What If Your Belly Is Measuring 'Big' or 'Small?'

The main concern with measuring bigger or smaller than expected is that your baby isn't growing properly. A fundal height that is measuring "big" or "small" can suggest that the fetus is bigger or smaller than expected (health care providers call these situations "large for gestational age" or "small for gestational age") or that there is too much or too little amniotic fluid (conditions known as polyhydramnios or oligohydramnios).

If your pregnant belly size isn't measuring as expected, your health care provider will likely watch your measurements more closely in the coming weeks to see if they even out. If not, they may order further tests to rule out other concerns.

Smaller than expected

A pregnant belly size that is smaller than expected could indicate intrauterine growth restriction (IUGR), which is also called fetal growth restriction. According to the Society for Maternal-Fetal Medicine, this complication occurs in around 10% of pregnancies. It can be caused by the following:

• Congenital or chromosomal anomalies
• Infections
• Vascular problems
• Nutritional deficiency

IUGR can result in complications, like poorer developmental and cognitive outcomes. Treatment for IUGR usually involves closely monitoring the rest of your pregnancy, which may include additional ultrasounds, non-stress tests, or even early delivery.

Larger than expected

A pregnant belly size that is larger than expected could suggest that the fetus is growing larger than expected and may be at risk for macrosomia, which is diagnosed in babies who weigh more than 4 kilograms (8.8 pounds) at birth.

The biggest concern with a fetus that grows too big is birth complications. For example, according to ACOG, a large fetus is more likely to result in:

• Labor abnormalities
• Shoulder dystocia
• Birth trauma
• Permanent injury to the newborn

The most common cause of fetal macrosomia is maternal diabetes. If ongoing monitoring indicates macrosomia, a health care provider may recommend a C-section to prevent possible birth complications.

If a health care provider is concerned about your belly, they will order an ultrasound to check on your fetus. The ultrasound will measure the circumference around your baby's stomach and head and the length of their legs to see if there may be a problem. They will also likely check for gestational diabetes with a glucose tolerance test if you haven't already had one.

If you have gestational diabetes, you may need to cut out excess sugar in your diet and, if necessary, take medication. Although gestational diabetes can be serious if left unchecked, most people with the condition give birth to healthy babies, and after that, it usually goes away.

What Pregnant Belly Size Doesn't Mean

Friends, family, and even total strangers love to comment on people's pregnant belly size ("Wow, you're so big! Sure it's just one baby?"), and there is no shortage of old wives' tales about what the size and appearance of a person's baby bump mean (Carrying high? "Get ready for a little girl!").

While the size of your pregnant belly can give your health care provider important clues about the health of your pregnancy, it won't tell anyone how many babies you're carrying, the sex of the baby, or even when you'll go into labor. In fact, what your baby bump looks like has more to do with your body shape, height, and strength of your abdominal muscles than anything else.

Everyone's body is different, and by extension, so are pregnant bellies. It's your health care provider's job to monitor your pregnancy, so if they're not concerned about your belly size, no one else should be.