I have a “small baby” – what does this mean?

What do obstetricians mean by a “small” baby?

A baby whose birth-weight is in the lowest 10% of the population is called “small”. Or to put it another way, the baby weighed less than 90% of its newborn peers.

What causes a baby to be small?

1. About 2/3 of small babies are simply “constitutionally small”. These babies are absolutely healthy. They grew appropriately during the pregnancy but were always destined to weigh 6 pounds rather than 10 pounds. Just as there is a range of different heights for adults, there is a range of healthy sizes for babies.

2. A rarer cause, but potentially more serious, is called fetal growth restriction (or intrauterine growth restriction). A growth-restricted baby is not thriving in the womb. Usually, this occurs because the placenta (which supplies blood to the growing fetus) is not working 100%. We monitor babies with fetal growth restriction very carefully, because they are at risk of serious complications.

3. 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.

4. Rarely, babies are small because of an underlying chromosomal problem or genetic problem.

Do you often see couples who had a small baby in a previous pregnancy?

Yes – approximately 1-in-10 couples will have had a small baby in an earlier pregnancy. This is one of the common reasons that couples come to see me.

My previous baby was growth restricted – is my current pregnancy at higher risk?

Yes. You have an increased risk of fetal growth restriction in all future pregnancies. This is especially the case if the growth problem was severe enough to need early delivery of the baby. Women who needed early delivery in their last pregnancy should seriously consider seeing a Maternal-Fetal Medicine specialist.

I read that some medical problems are linked to small babies?

This is correct. Some underlying medical problems interfere with the development and function of the placenta. Conditions such as high blood pressure, kidney disease and some autoimmune conditions all increase the risk of fetal growth restriction, as do pregnancy complications like pre-eclampsia.

This risk can be reduced by optimising the management of your medical condition before pregnancy. Some women also benefit from low-dose aspirin started in early pregnancy.

For this reason, I strongly recommend pre-pregnancy counselling to any woman with a pre-existing medical condition, a history of pre-eclampsia or a previous small baby.

What additional tests do you recommend in my current pregnancy?

The most important test is regular ultrasound scanning 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).

Women with a previous small baby should at least have specialist growth scans at 28-30 weeks and 34-36 weeks.  This is in addition to the routine scans at 12 and 19 weeks.

The bedside scans performed by most obstetricians are not specialist growth scans. General obstetricians do not have the training, experience or qualifications to perform diagnostic ultrasound scans. As a Maternal-Fetal Medicine specialist, I do have the training and experience to perform diagnostic growth scans and Doppler studies myself. I perform specialist scans for all my private patients at their antenatal appointments.

Depending on your circumstances and what we find on ultrasound, other tests may be recommended. This may include testing for pregnancy complications (like pre-eclampsia), specific infections, or a specialised chromosome test called an amniocentesis.


What else can I do to avoid my baby being small?

It’s important to avoid lifestyle choices that harm the placenta. Smoking in pregnancy is a very significant risk factor for fetal growth restriction. If you are a smoker, please talk to your GP as soon as possible for help quitting. The same goes for recreational drugs.

As always, this information is intended for general educational purposes only. It is not medical advice. Please discuss any medical issues with your own doctor. Read our full medical disclaimer here.