Find out what measuring small for your due date means for your baby
Measuring your abdomen is a simple prenatal test your health care provider can use to estimate the size of the baby. After 20 weeks, the distance from the pubic bone up over the top of the uterus is on average about the same number of centimeters as weeks' gestation, give or take 4 centimeters. If the number is smaller than expected, we typically evaluate the baby with a prenatal test such as ultrasound to see how it is growing. Here are some scenarios that may explain a smaller-than-expected tummy:
1. The pregnancy is not as far along as you thought.
But since the earliest ultrasound is the most accurate for dating, if you already had an ultrasound this is unlikely to be the cause.
2. Sometimes the tummy measures small but the measurements of the baby are fine.
I usually attribute this to the way you are carrying the baby, which has no medical significance.
3. The baby is symmetrically small.
In this case, 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.
4. The baby's head is growing OK, but the abdomen and maybe the leg bone are behind.
This is called asymmetrical intrauterine growth restriction (IUGR), and is usually caused by inadequate nutrients getting to the baby. To 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 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, 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 more closely than normal, and will make recommendations to optimize your baby's health.