Being told your baby may be "small for dates" is worrying. In many cases it turns out that the baby is simply small and perfectly healthy. In some cases it means the baby is not growing as well as it should, and that is something we watch closely. This page explains the difference, and what happens next.

Two different things: small, and not growing well

These sound similar but are not the same, and the distinction matters.

Small for gestational age (SGA) means the baby's estimated weight is below the 10th centile for the number of weeks of pregnancy — in other words, smaller than nine out of ten babies at the same stage.1

Fetal growth restriction (FGR) means the baby is not reaching its own growth potential — usually because the placenta is not delivering enough oxygen and nutrients.1

The two overlap but are not identical:

  • Many SGA babies are constitutionally small. Small parents often have small babies. These babies are healthy and simply built that way.
  • Some babies whose weight is in the normal range are still growth restricted, because they should have been bigger.

That is why the pattern of growth over time, and the blood-flow measurements, matter more than a single weight estimate.

Why it matters

Babies who are genuinely growth restricted have a higher chance of problems before birth, during labour, and in the newborn period. Identifying them allows us to monitor closely and to plan the timing and manner of birth — which is where the benefit lies.1

The purpose of monitoring is not to alarm you. It is to catch the babies who need extra attention and to reassure the far larger number who do not.

What raises the chance

Your risk is assessed at your first visit and reviewed as pregnancy goes on.1 Factors that raise it include:

  • A previous small baby, or a previous stillbirth
  • High blood pressure in pregnancy, or pre-eclampsia
  • Smoking, and exposure to other people's smoke
  • Long-standing medical conditions — kidney disease, autoimmune conditions, poorly controlled diabetes
  • Certain infections in pregnancy
  • Twins or triplets
  • Bleeding in pregnancy, or problems with the placenta
  • Mother's age over 40
  • Poor nutrition and low weight gain in pregnancy

Many women with none of these still have a small baby, and many with several risk factors have a perfectly grown one. Risk factors guide how closely we look; they do not predict the outcome.

How growth is monitored in Sri Lanka

Monitoring happens at two levels, and both matter.

In the field — symphysis-fundal height

At routine antenatal visits in the community, growth is followed by measuring the symphysis-fundal height — the distance from the pubic bone to the top of the womb, in centimetres, plotted on a chart. It is a simple measurement, it is done at every visit from around 24 weeks, and it is the mainstay of routine screening.2

If the measurement is smaller than expected, or is not increasing as it should, you will be referred for a scan.

At the clinic — serial growth scans

Where closer surveillance is needed, growth is followed with ultrasound scans repeated every 4 weeks, measuring the baby's head, abdomen and thigh bone to estimate weight, together with the amniotic fluid index — the volume of fluid around the baby, which falls when the placenta is under strain.3

Scans are spaced at least a few weeks apart for a good reason: measured too close together, the natural error of the measurement is larger than the real growth, and the result can be misleading.

Doppler studies — only where SGA is suspected

Doppler scans measure blood flow rather than size.

  • Umbilical artery Doppler looks at flow in the cord, and is the single most useful test for deciding whether a small baby is coping.1
  • Middle cerebral artery Doppler looks at flow in the baby's brain, and adds information in some situations.

These are not routine tests for every pregnancy. They are used when a baby is suspected of being small or growth restricted — which is exactly when they give useful information.

If your baby is found to be small

The plan depends on how small, how the blood flow looks, and how far along you are.

  • More frequent monitoring — closer scans and Doppler studies, so that any change is picked up early
  • Looking for a cause — blood pressure, urine testing, and other investigations where indicated
  • Steroid injections, if early delivery becomes likely, to help the baby's lungs mature
  • Planning the birth — deciding together when and how the baby is best delivered

Timing is a balance: staying in the womb has benefits, but so does being born if the placenta is failing. That balance is judged individually, and it can change from week to week. There is no fixed rule that applies to everyone.

What you can do

  • Stop smoking. This is the single most effective thing within your control, and it helps at any stage of pregnancy.
  • Attend every antenatal appointment. The fundal height measurement only works if it is done repeatedly.
  • Eat well and gain weight steadily. See our article on nutrition in pregnancy.
  • Take the supplements you have been advised — iron and calcium as recommended.
  • If you have been prescribed low-dose aspirin because you are at higher risk of pre-eclampsia, take it as directed. This is prescribed for some women, not all — do not start it on your own.2
  • Get medical conditions well controlled, ideally before pregnancy.

When to seek help urgently

Contact your doctor or maternity unit straight away — do not wait for your next appointment — if you notice:

  • Reduced or changed baby movements. This is the most important warning sign in the whole of pregnancy. Never wait to see if it improves.
  • Vaginal bleeding
  • Severe headache, visual disturbance, or sudden swelling of the face and hands
  • Abdominal pain that does not settle
  • Fluid leaking from the vagina

The takeaway

Most small babies are simply small, and healthy. The purpose of measuring at every visit, scanning where needed, and using Doppler studies when a baby is suspected of being small is to find the smaller number of babies who are genuinely not growing well — early enough to do something about it. Attend your appointments, report any change in movements immediately, and ask if anything is unclear.

References

  1. Morris RK, Johnstone E, Lees C, Morton V, Smith G, on behalf of the Royal College of Obstetricians and Gynaecologists. Investigation and Care of a Small-for-Gestational-Age Fetus and a Growth Restricted Fetus (Green-top Guideline No. 31). BJOG. 2024;131(9):e31–e80. Available from: https://www.rcog.org.uk/guidance/browse-all-guidance/green-top-guidelines/small-for-gestational-age-fetus-and-a-growth-restricted-fetus-investigation-and-care-green-top-guideline-no-31/
  2. Royal College of Obstetricians and Gynaecologists. Having a small baby [Internet]. London: RCOG; [cited 2026 Jul 22]. Available from: https://www.rcog.org.uk/for-the-public/browse-our-patient-information/having-a-small-baby/
  3. NHS. Ultrasound scans in pregnancy [Internet]. London: NHS; [cited 2026 Jul 22]. Available from: https://www.nhs.uk/pregnancy/your-pregnancy-care/ultrasound-scans/