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Congratulations, you’re expecting! This exciting and sometimes nerve-wracking time can mean browsing dream strollers, finding fashionable maternity wear — and lots of doctors’ appointments.

Regular doctor checkups and tests during pregnancy are important for your health and your baby’s too. If you’re a pregnant person anywhere in the world, you’ll probably have your belly measured several times during your pregnancy.

As your belly starts to blossom, your doctor will measure your growing bump regularly with a measuring tape. This measurement is called fundal height.

Here’s more on what fundal height means and why it’s an important part of your pregnancy checkups.

Fundal height is a vertical (up and down) measurement of your belly. It’s the distance from the pubic bone to the top of your womb (uterus).

Your doctor might also call it the symphysis-fundal height (SFH). Symphysis is the scientific name for bones that are joined together, like in the pelvis.

Fundal height is always measured in centimeters. To begin, you’ll lay flat on your back on the exam table, just like you’re getting an ultrasound scan.

Your doctor will use a flexible measuring tape to measure the distance from your pelvis area to the top of your uterus. This is about where your belly slopes downwards after going over the highest point of your bump.

Around 24 weeks of pregnancy, the fundal height usually matches the number of weeks you’ve been pregnant. This quick measurement is not just a really good party trick —measuring the fundal height can help tell a few important things about your pregnancy:

  • Your doctor will have a better estimate how far along you are in the pregnancy (gestational age).
  • Your doctor can chart fundal height over time to make sure you’re gaining weight and your baby is growing consistently.
  • Fundal height helps to show how large your baby is (fetal growth) because your bun in the oven is making that belly bump after all!

So if your fundal distance is about 26 centimeters (plus or minus 2 centimeters), you are probably around week 26 of your pregnancy. Of course the fundal height can’t give an exact number of weeks or show the exact size of your baby, but it’s a good estimate.

Measuring the fundal height is especially important in places where there isn’t an ultrasound machine available or if you’re at a checkup at your doctor’s office between ultrasound scan appointments.

The fundal height is used by doctors around the world to help check both mama’s pregnancy health and baby’s growth easily and quickly and without needing any expensive, high-tech equipment.

The fundal height is so important that researchers made an international formula to help better estimate pregnancy weeks and baby size with this measurement.

The formula takes into account that measuring a pregnant person’s stomach won’t always be accurate, and some mothers-to-be might show more or have a bigger belly than others.

If you’re in week 25 of your pregnancy but your fundal height measurement estimates that you’re in week 28 or 29, don’t worry. There are several reasons why this might happen:

  • You may have been pregnant earlier than you or your doctor calculated according to your missed period cycle or first ultrasound.
  • Your baby might be bigger than average.
  • Your body shape or type might just mean that your stomach is sticking out a bit more.
  • You might have a full bladder or really need a bowel movement. Yes, both these things can sometimes make your stomach stick out about 3 centimeters higher!

If your fundal height is too long, your doctor will double-check the measurement and may also give you an ultrasound scan. Some babies are just longer or bigger than average.

If your baby is larger than other babies at that stage in pregnancy, it might be due to:

  • Genetics: You, your partner, or both of you may have tall or large genes in your family that are passed down to your baby.
  • Weight gain: If you’ve gained excess weight during your pregnancy or before pregnancy, your baby may gain more weight too.
  • Diabetes: If you have gestational diabetes or another kind of diabetes, your baby might be larger than usual.

Sometimes fundal height is shorter than it should be and doesn’t match up with how far along your pregnancy is. There are several reasons why this might happen:

  • Your pregnancy might have happened later than you or your doctor estimated. For example, you might estimate that you’re in week 28 of your pregnancy, but you’re actually in week 26 or 27.
  • Your baby might be smaller than normal.
  • Your body shape or type might just mean that your belly doesn’t show as much.

Your doctor may be more concerned if your baby is smaller than normal or is not gaining weight. A lack of weight gain in utero may lead to low birth weight.

Causes of small babies in the womb include:

  • Genetics: You, your partner, or both of you may have shorter or small genes in the family.
  • Genetic conditions: Certain chromosomal conditions can cause smaller babies early on in the pregnancy.
  • Nutrition: Your baby may not be getting as much oxygen, blood flow, and nutrients as they should be.
  • Uterine issues: A problem with the placenta might be limiting how much blood and oxygen your baby is getting.
  • Mama’s nutrition: If you aren’t taking in adequate healthy foods, or have a body weight of under 100 pounds, baby may not grow as quickly as hoped either.
  • Alcohol and drugs: Alcohol, cigarette smoking, or drug use can all affect your baby’s growth and development.
  • Mama’s health: Your health matters for your baby’s health and weight gain. A number of conditions that you might have can affect your baby’s health and growth. These include:

Checking your fundal height is just one way that your healthcare provider can check your pregnancy health and your baby’s growth and development.

It’s not always accurate, but along with ultrasound scans and other tests, measuring the fundal height can help keep your pregnancy and baby healthy.