As your pregnancy progresses, you might compare your baby’s size to the fruit or vegetable recommended that week in your pregnancy app. Your doctor, though, has a few more accurate ways of gauging your baby’s size to help ensure your pregnancy is going as expected.
Here’s how your practitioner will measure your fundal height and your fetus size throughout pregnancy, and why these measurements are sometimes a bit bigger or smaller than expected.
What is fundal height?
Fundal height is the measurement in centimeters from your pubic bone to the top of your uterus. It’s used to help estimate your baby’s length and weight. Your doctor will compare your fundal height to the average fundal height of moms at the same week of pregnancy.
Most of the time, fundal height gives you extra peace of mind that your baby’s doing well. Since measuring your uterus from the outside isn’t a precise science, it’s usually okay if your fundal height doesn’t correlate exactly with your estimated due date.
How to measure fundal height: How does my doctor measure the size of my fetus in utero?
The simplest way to gauge a baby’s size in utero is to measure an expecting mom’s fundal height. Fundal height measures the distance from the pubic bone to the top of the uterus in centimeters. Your health care practitioner will also palpate your abdomen to get an idea of your baby’s size.
After 24 weeks of pregnancy, the fundal height in centimeters roughly corresponds to the week of gestation. That means if you’re 29 weeks pregnant, your fundal height is probably about 29 centimeters.
Between weeks 37 and 40, your baby starts to descend into your pelvis in preparation for birth, so the fundal height measurement begins to decrease.
More About Fetal Development
Transvaginal and transabdominal ultrasounds are another more accurate — but still not foolproof — way that doctors estimate a baby’s size.
Here's how: An ultrasound wand sends sound waves that bounce off of your baby to produce the images you see on the screen. A sonographer will take various measurements of your baby and then use them to estimate your baby’s weight by plugging them into an algorithm.
When will my practitioner measure fundal height?
Doctors begin measuring fundal height at around week 20 of pregnancy and at every prenatal appointment until the baby is born.
Ultrasound can also help estimate your baby's weight. In the first trimester, ultrasound is used to estimate your baby's crown-to-rump length (the distance between the top of his head to his bottom) and, consequently, your due date. In the third trimester, your practitioner will use ultrasound to measure various parts of your baby's body, estimate his size and weight and ensure that everything is progressing as expected.
As you reach the end of your pregnancy, you may want to know exactly how big your baby will be at delivery. But there’s no reason to have an extra ultrasound to estimate the size of your baby if your fundal height is on track.
Although ultrasounds are generally accurate, they can over- or underestimate a baby’s size by 10 to 20 percent, which may in rare cases lead to unnecessary interventions like C-sections. And while ultrasounds are very safe, doctors recommend against unnecessary ultrasounds during pregnancy.
How accurate are measurements of my baby's size in utero?
Any estimation of your baby’s size in utero is simply a best guess based on averages. Every mom is a different size and shape, as is every baby inside. Measuring bigger or smaller than the average is usually perfectly normal. In fact, research suggests fundal height measurements are often off by two weeks in non-obese women.
There’s always a margin of error when it comes to estimating the exact weight and size of an unborn baby. Estimates of your baby’s size can be off by a pound or more, and being a centimeter or two bigger or smaller than the expected measurement is usually no cause for concern.
Fundal height can be less accurate if you’re obese or have a history of uterine fibroids. You may measure off by a week or two because of other factors, including your baby’s position and the volume of amniotic fluid on that given day. Sometimes a baby may measure large or small because your estimated due date isn’t accurate.
What your doctor will do if you measure big or small
Any estimation of your baby’s size in utero is simply a best guess based on averages. Every mom is a different size and shape, as is every baby inside. Measuring bigger or smaller than the average is usually perfectly normal. In fact, research suggests fundal height measurements are often off by two weeks in non-obese women.
There’s always a margin of error when it comes to estimating the exact weight and size of an unborn baby. Estimates of your baby’s size can be off by a pound or more, and being a centimeter or two bigger or smaller than the expected measurement is usually no cause for concern.
Fundal height can be less accurate if you’re obese or have a history of uterine fibroids. You may measure off by a week or two because of other factors, including your baby’s position and the volume of amniotic fluid on that given day. Sometimes a baby may measure large or small because your estimated due date isn’t accurate.
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