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.