I recently wrote about how we measure the uterus during pregnancy. So what does it mean if your "fundal height" (the name for that measurement) is smaller than expected?
First of all, only focus on measuring big or small if your doctor or midwife is concerned, or you feel a lot different than you did in a prior pregnancy. People in your life may feel the need to comment on your size and shape when you are pregnant, but this doesn't mean anything unless you or your practitioner are also concerned.
If you do measure smaller than expected, several factors may be playing a role.
The measurement of the abdomen during prenatal visits is a very rough estimate of the size of the baby. If the fundal height is smaller than expected, we typically evaluate the baby with ultrasound to see how it is growing. Ultrasound isn't perfect -- estimated weight is actually give or take 15%, which amounts to over a pound at term, but it is the most scientific way we have to estimate fetal growth.
Here are some scenarios that may explain a smaller-than-expected tummy.
- Are you "less pregnant" than you thought? (This is unlikely if you already had an ultrasound that confirmed a due date.)
- Is this just normal for you? Some women carry their babies low, either in one pregnancy or in all their pregnancies. Since the measurement of fundal height only takes the up-down dimension into account, a baby that is laying sideways or that is low in the mother's pelvis may measure small.
- Is the baby small? If the baby does seem small on ultrasound, they will try to determine if the baby is symmetrically small, or if the head is growing normally but the abdomen is falling off the growth curve.
- When the baby is symmetrically small, the baby's head, abdomen, and leg bone are all measuring less than expected for gestational age. This may be normal in your family, if people aren't that big. Your doctor will tell you more about symmetrical growth restriction if that turns out to be the case.
- When the baby is asymmetrically small, the head will be found to be growing OK, but the abdomen and maybe the leg bone are behind. This is called asymmetrical intrauterine growth restriction (IUGR), and may be caused by inadequate nutrients getting to the baby.
The baby's physiology tries to compensate and protect brain development. Nutrients are sent to the head, and brain growth continues even if the rest of the baby isn't growing that well, so the abdominal measurements typically lag behind the head.
Asymmetrical IUGR can be caused by poor blood flow to the placenta, as is sometimes seen with cigarette smoking, maternal high blood pressure, or other vascular problems. Typically the doctor will take measurements with a special doppler ultrasound to check blood flow to the uterus. Poor maternal nutrition can also cause the baby to be small.
If your baby is smaller than expected, you will probably be followed more closely than usual with ultrasounds, fetal monitoring (non-stress testing) and/or daily fetal movement counts. If your nutrition is a problem, consulting with a dietitian and increasing your intake may help. If the problem is blood flow, resting on your side several hours a day may improve growth.
Sometimes it is best to deliver the baby early so he or she can start to eat instead of being dependent on a placenta that isn't working well. Your medical team will follow your pregnancy closely, and will make recommendations to optimize your baby's health.