What does “small” mean?
As a pregnancy progresses, various measurements of the fetus (head, waist and leg) are taken during an ultrasound scan to determine if the baby is growing appropriately. These measurements are compared to a chart of averages to determine if your baby is measuring average, big or small (or rarely, very small or very big). Babies who are measuring in the bottom 10% for that particular number of weeks (called the 10th percentile), are called “small for gestational age (SGA)”.
What are the possible causes of my baby measuring small?
- A common reason for your baby to measure small is if there is uncertainty regarding your due date. Ultrasound scans before 14 weeks are very accurate at determining due dates – women who have had an early scan can be pretty certain that their dates are accurate.
- The most common reason (about 2/3s) for small babies is simply called “constitutionally-small”. This means that your baby is perfectly healthy but was never destined to weigh 9 pounds, in much the same way as watching people walking by on the street reveals a range of normal heights.
- A rarer cause, but potentially more serious, is called fetal growth restriction (or intrauterine growth restriction), where the baby is not thriving in the womb. This can only be ascertained over several weeks, where the trend in baby’s growth becomes apparent. Usually, fetal growth restriction occurs because the placenta (which supplies blood to the growing fetus) is not working 100%.
- In a small number of cases, babies can measure small because the mother picked up an infection earlier in the pregnancy. Many infections (such as CMV or toxoplasmosis) cause only a mild flu-like illness for the woman but can have serious consequences for fetal development.
- The last cause of SGA is babies who are small for some other reason. This includes chromosomal problems or genetic problems.
What other tests should I consider for my baby?
The most important test for small babies is regular growth scans during the remainder of the pregnancy, to carefully monitor the baby’s growth. The ultrasound also allows us to measure the blood flow in the placenta (called fetal Doppler studies) and the amniotic fluid volume (waters). For babies who are very small (<3rd centile) or where other structural problems are identified on scan, a specialised chromosome test, called an amniocentesis, should be considered. At SHORE FOR WOMEN, we are very experienced in caring for small babies, including monitoring with advanced fetal growth and Doppler scans.