Pregnant women learn a lot about their bodies in those nine months than most men learn about theirs in their entire lives. Between conception and birth, all those exams and tests in the OB's office make us acquainted with such odd pregnancy body phenomena as mucus plugs and pelvic separation. Then there's the issue of fundal height and what it means for the pregnancy, a topic no one ever thinks about unless they're either expecting or cramming for exams in med school.
Mysterious as it sounds, it's actually a pretty simple concept: The uterus gradually expands like a balloon as the pregnancy progresses. The farther along in your pregnancy, the larger your uterus should be, the better to give the baby room to grow and develop. By the third trimester, the uterus reaches its full size, which is roughly the size of a watermelon (oof!).
In the second trimester, your doctor may begin measuring the distance between your pubic bone and the top of your uterus (also known as the fundus). The size of the fundal height in centimeters should closely match the number of weeks of pregnancy, per Mayo Clinic. In other words, a woman in her 30th week should have a fundal height somewhere around 29 to 31 centimeters.
There are a host of things that your uterus has to say about the health of you and your baby during this time. Of course, you should always discuss any concerns with your OB or other health professional, but knowing what the fundal height measurement means can help you determine what steps to take if the measurement shows signs of abnormality. Read on to learn what this important maternal test means. For instance, it can tell you...
1. When You'll Give Birth
Because the size of the uterus corresponds with the size of the baby, knowing your fundal height can help the doctor figure out how far along you are and when you might give birth. If the uterus is a little bigger than expected, it may mean that your due date was miscalculated and you're actually a week or two farther along, explained BabyCenter. However, there are multiple factors that may affect the fundal height, cautioned the Mayo Clinic, so don't look at the number as an exact predictor.
2. You May Have Fibroids
If your fundal height indicates you're "large for your gestational age," there may be something other than a baby that accounts for the size of your belly. One possible cause: fibroids. Per the Fibroid Treatment Collective, these uterine growths are most often benign and, depending on their position, have no effect on the pregnancy or the health of the baby. They can, however, increase the size of the uterus, thus increasing the fundal height and making it seem as though you're farther along than you are. And some uterine fibroids are positioned in such a way that they block the birth canal or lie too close to the placenta, which can deprive the baby of nutrients. The collective added that having fibroids isn't necessarily dangerous in itself, so don't be alarmed if you get this diagnosis. (However, fibroids can affect fertility, so if you're hoping to add to your family, the collective recommends trying to conceive as soon as possible after having your baby.) In a nutshell, it's best to have a conversation with your doctor.
3. You May Be Having Multiples
Surprise! One of the possible reasons for an unusually large fundal height is that you're carrying twins (or even more than two babies), according to Babble. Naturally, having an extra body growing inside your uterus will make it expand more quickly than if you were having just one baby. Your doctor will take this into consideration at future exams.
4. Your Baby Could Be Breech
Some babies decide to make themselves comfy in the feet-downward breech position, which occurs in about 4 percent of all full-term births, according to the American College of Obstetricians and Gynecologists (ACOG). Because the largest part of the baby, the head, is positioned at the top of the fundus, this can cause a higher-than-expected fundal height reading. In the case of a breech position, your OB will discuss such options as attempting to turn the baby externally or delivering via c-section to avoid the risks of a vaginal breech delivery.
5. You May Have Gestational Diabetes
As in the case of twins, carrying a bigger baby may result in a higher than anticipated fundal reading. ACOG noted that women with gestational diabetes may have larger babies because their higher blood-sugar levels are passed on to the fetus. It's typical for expectant moms to be screened for gestational diabetes at some point in the second trimester, but women with risk factors for diabetes should be screened earlier.
6. Your Amniotic Fluid May Be Low
On the other end of the spectrum, a fundal reading that's much smaller than expected for the gestational age can mean that the baby is growing more slowly than it should. One reason for this, explained WebMD, is a lower-than-normal level of amniotic fluid. (The medical term for this is oligohydramnios, noted Babble; try saying that three times fast.) An ultrasound can measure fluid levels throughout the pregnancy, and if yours are low, your doctor will keep a close watch on you and the baby to make sure all is well.
Keep in mind that fundal height is only one of the ways your care provider monitors your growing baby's health. Nor is it the last word in diagnostics, warned the Mayo Clinic. Ultrasounds give a much more accurate picture of fetal growth, and they can detect such problems as low amniotic fluid. Still, if your fundal height measurement indicates that your uterus is growing at a normal rate, that's one less worry off your hands (or stomach).
Editor's note: This post has been updated from its original version.