Small for Gestational Age

Small is a measurement. Growth restriction is a diagnosis.

Green-top Guideline No. 31 defines the first: "Commonly, the definition of SGA refers to a fetus with a predicted weight or an abdominal circumference (AC) measurement less than the 10th centile." The RCOG's patient information says the same in plainer terms: "If the estimated weight of your baby by ultrasound scan is in the lowest 10% of babies (the smallest 10 out of every 100 babies, or less than the 10th centile), they are considered to be small."

The second is different. "Fetal growth restriction (FGR) implies a pathological restriction of the genetic growth potential. Some, but not all, growth restricted fetuses/infants are SGA. The likelihood of FGR is higher in fetuses that are smaller."

By definition, one in ten babies is below the 10th centile, and most of them are simply small. The RCOG puts it this way: "Most babies do not have any health problems but are just smaller than others and this is normal for them." The entire purpose of the tests described below is to separate the constitutionally small baby from the one that is not growing as it should.

The guideline is candid that this is hard: "Defining FGR and thus diagnosing it in a current pregnancy is challenging because of the need to determine growth potential."

How you got here

NICE guideline NG201 sets out the routine pathway. Recommendation 1.2.29 asks for "a risk assessment for fetal growth restriction at the first antenatal (booking) appointment, and again in the second trimester". Recommendation 1.2.30 asks for symphysis fundal height measurement "at each antenatal appointment after 24+0 weeks (but no more frequently than every 2 weeks)" unless you are already having regular growth scans, plotted on a growth chart.

Recommendation 1.2.32 is the trigger: "If there are concerns that the symphysis fundal height is small for gestational age, offer an ultrasound scan for fetal growth and wellbeing, the urgency of which may depend on additional clinical findings, for example, reduced fetal movements or raised maternal blood pressure."

NICE also says not to offer routine scans after 28 weeks in uncomplicated singleton pregnancies (1.2.33), which is why you may have gone from tape measure straight to a first growth scan at 30-plus weeks.

If you are in the higher-risk group from the start, the RCOG says you will be offered either additional growth scans, or "a uterine artery Doppler scan - this is an ultrasound scan of the blood flow to your placenta. This is usually done at around 20 weeks."

The charts changed in 2025 - and it may affect you

This is recent enough that many people have not been told. The RCOG records the following update to Green-top Guideline No. 31: "In December 2025 NHS England asked all maternity units in England to stop using Intergrowth 21 estimated fetal weight (EFW) charts (2000 or 2017 versions) due to concerns about their applicability in the NHS population. Analysis indicated that Intergrowth EFW charts could lead to a lower screen positive rate of small-for-gestational-age fetuses (<10th centile). This could mean that some fetuses that otherwise would have undergone enhanced surveillance had not."

In practice: a scan reported as normal on the old chart could be reported as below the 10th centile on a different one. If you have had scans across that change, it is reasonable to ask which chart your measurements were plotted on.

The tests that follow

What happens next depends on how small your baby is and how early it was noticed. The RCOG lists the options:

• Umbilical artery Doppler - "an ultrasound scan which measures the flow of blood through the umbilical cord. It can help to tell whether your baby is at risk of becoming unwell and whether they may need to be born early." This is the central test.
• More frequent growth scans.
• Additional Dopplers checking blood flow in the baby's brain and abdomen.
• A computerised cardiotocograph (cCTG) - a tracing of your baby's heart rate.

Referral to a fetal medicine specialist is offered "if your baby is very small, they have been found to be small early in the pregnancy, or the umbilical artery Doppler scan is not normal".

Green-top Guideline No. 31 explains why this battery exists rather than a single measurement: "There is a need to focus on those fetuses at risk of adverse outcome and thus those that are FGR rather than SGA using varying parameters such as sequential ultrasound measurements, Doppler assessments, and biomarkers." Growth restricted babies "may manifest evidence of fetal compromise (abnormal Doppler studies, reduced liquor volume)".

What being small means for your baby

The RCOG separates the outcomes honestly: "If your baby is small and healthy, they will have a lower chance of complications than if they are small and are known to have an underlying health problem. The earlier in pregnancy and the more severely your baby's growth is affected, the more likely it is that they will have complications. Babies whose growth is only affected later in pregnancy are less likely to have severe complications."

Where growth restriction is present, the risks named are stillbirth, being unwell at birth, dying shortly after birth, preterm birth and neonatal unit care.

What raises the chance

From the RCOG: being aged 40 or more; being underweight (BMI under 18.5); smoking or drug misuse, particularly cocaine; more than the recommended caffeine intake; health conditions including high blood pressure, diabetes, kidney disease, auto-immune disease, complicated heart disease and blood clotting problems; a previous small baby; a previous pregnancy complicated by blood pressure problems or pre-eclampsia; a previous stillbirth. Heavy vaginal bleeding, especially in the second half of pregnancy, can also affect growth.

Two of these come with numbers attached. On smoking: "If you can stop smoking by the time you are 15 weeks pregnant your chance of having a small baby will reduce to being similar to that of a non-smoking woman." On caffeine: "the current recommended maximum intake is 200mg per day, for example, two mugs of instant coffee."

And on aspirin: "If you are at increased risk of having problems with your placenta or of developing pre-eclampsia, you should be advised to take low-dose aspirin (150 mg once each evening) from 12 weeks of pregnancy until 36 weeks of pregnancy." Note the timing - this is prevention started early, not a treatment begun after a small baby is found.

Timing and mode of birth

Where growth restriction is confirmed and the baby is compromised, NICE guideline NG207 is categorical: "Do not induce labour if there is fetal growth restriction with confirmed fetal compromise. Offer caesarean birth instead."

Where there is no confirmed compromise, the decision is a balance between the risks of staying in and the risks of being born early, informed by the Doppler results, the CTG and how the growth is trending over successive scans rather than any single figure. The RCOG's framing is that you "may be advised to give birth before your due date" - advice you are entitled to have explained in terms of your own scan results.

Ask three questions at each scan: what centile is my baby on, is the trend flat or falling, and what is the umbilical artery Doppler doing. Those three answers, taken together, are what the decisions are actually being made on.