Baby Measuring Small or Large? Here’s What It Means and What Happens Next
By Dr Greg Jenkins | Sydney-based specialist in Fertility, Obstetrics and Gynaecology
During pregnancy, one of the most common questions parents ask is, “Is my baby growing as expected?” Ultrasound scans give an estimate of your baby’s size and growth, but variations are common. A baby measuring smaller or larger than average does not automatically mean something is wrong. It simply prompts your doctor to look a little closer.
Understanding Growth Scans
Growth scans are ultrasound examinations that measure your baby’s head, abdomen and femur length to estimate weight and track development. These measurements are compared to population averages to calculate a percentile, a way of showing how your baby’s size relates to others at the same stage.
Generally, babies between the 10th and 90th percentiles are considered within a normal range. Some naturally fall on the smaller or larger side, often influenced by parental genetics, health, and other factors. When a baby measures outside this range, it triggers closer review rather than immediate concern.
When Baby Measures Small
A baby below the 10th percentile is described as small for gestational age (SGA). In many cases, this simply means your baby is constitutionally small, perfectly healthy but naturally smaller in build. However, sometimes it may indicate fetal growth restriction (FGR), where the baby is not growing as expected inside the womb.
Common Causes Include:
Placental insufficiency, where the placenta is not transferring enough oxygen or nutrients.
Maternal health factors such as high blood pressure, preeclampsia, or smoking.
Multiple pregnancy, where twins share nutrients differently.
Genetic or chromosomal conditions, which are much less common.
What Happens Next:
If your baby measures small, your obstetrician will usually recommend:
More frequent scans to monitor growth patterns and amniotic fluid levels.
Doppler studies to assess blood flow in the umbilical cord and placenta.
Maternal health checks for blood pressure, iron, and other key indicators.
Careful timing of delivery, as sometimes it is safer to deliver a bit earlier if the baby’s growth slows significantly.
Most small babies are born healthy. With attentive monitoring, early detection helps ensure the best possible outcome.
When Baby Measures Large
A baby above the 90th percentile is described as large for gestational age (LGA) or sometimes as having macrosomia. Like smaller babies, many large babies are perfectly healthy. Size can reflect family traits, since taller parents often have bigger babies. However, it may also be linked to specific conditions.
Common Causes Include:
Genetics, where larger size runs in families.
Gestational diabetes, which increases nutrient flow to the baby.
Post-term pregnancy, where growth continues beyond 40 weeks.
Excessive weight gain during pregnancy.
What Happens Next:
If your baby is measuring large, we assess:
Maternal glucose levels to check for gestational diabetes.
Growth trends, because ultrasound estimates can vary by up to 15 percent.
Sometimes, if a baby appears very large (above 95th per centile), your care team will discuss delivery options, including whether a vaginal birth is safe or if a caesarean may be recommended to avoid complications such as shoulder dystocia, when the baby’s shoulders become stuck during birth.
The Limits of Estimation
Ultrasound growth estimates are incredibly useful but not exact. A baby thought to be small or large can often be born perfectly average in weight. The goal of monitoring is to identify trends rather than rely on one measurement. Your doctor uses these patterns, along with your overall health and pregnancy progress, to make the safest plan.
Personalised Care Makes the Difference
Every pregnancy is unique. Some babies grow steadily on a smaller curve, while others stay large from early on. What matters is consistent growth over time and ensuring both baby and mother remain well.
As an obstetrician, I focus on combining accurate monitoring with calm, clear communication. When growth variations appear, we discuss the findings together, review the possible causes and create a plan that keeps both safety and peace of mind in balance. With the right care, most of these pregnancies proceed smoothly and end with healthy outcomes.
Meet Dr. Greg Jenkins: Your Partner in Fertility and Obstetrics
Dr Greg Jenkins is a specialist obstetrician, fertility specialist and gynaecologist, and the Head of Obstetrics and Gynaecology at Westmead Public Hospital. He provides personalised care for women and couples across fertility treatment, pregnancy and birth — with the unique advantage of continuity of care from conception through to delivery.
Dr Jenkins consults privately at Norwest Private Hospital in Bella Vista and works in partnership with Minerva Fertility at Bond Day Hospital. He offers delivery at Norwest Private, Westmead Private and Westmead Public Hospital, giving families flexibility in choosing the setting that best suits their needs.
