How Big Is My Baby?
Your baby grows from a poppy seed at week 4 to around 50 cm and about 3.4 kg at term. Early on, size is measured head-to-rump; from about week 20 it is measured head-to-heel.
Size by trimester
- First trimester: from a seed to about a lime (~8 cm) by week 12.
- Second trimester: banana to corn cob; the length measurement jumps at week 20 as it switches to head-to-heel.
- Third trimester: from a large melon to a small pumpkin, gaining fat and weight toward term.
How size is measured
Early scans measure the crown-rump length (CRL). Later scans use biometry (head, abdomen and femur) to estimate weight.
It varies
Every baby is different, and estimates have a margin of error. Your doctor looks at the trend over time, not one figure. See our week-by-week guide for the size at each stage.
Frequently asked questions
How is my baby's size estimated?
By ultrasound measurements of the head, abdomen and thigh bone, plus your dates.
Why did the length jump at 20 weeks?
Measurement switches from head-to-rump to head-to-heel around then, so the number rises.
Is my baby's size normal?
Most variations are normal; your doctor compares growth to a chart over time.
Follow your baby week by week 💜
See exactly how your baby grows in the complete 40-week guide.
40-Week GuideJoin The CircleReferences
- WHO. Antenatal care recommendations. who.int
- ACOG. Fetal development. acog.org
- NHS. You and your baby, week by week. nhs.uk
- Tommy's. Baby movements & development. tommys.org
- CDC. During Pregnancy. cdc.gov