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After the first trimester, ultrasound dates a pregnancy from the head (biparietal diameter BPD, head circumference HC), abdomen (AC) and thigh bone (femur length FL). Each measurement 'matches' a week of pregnancy; together they give the best estimate.
Accuracy falls as pregnancy goes on — about ±7–10 days before 20 weeks but ±2–3 weeks in the third trimester — so a late scan shows the baby's size more than the true dates.
| BPD | 82 mm |
| HC | 296 mm |
| AC | 280 mm |
| FL (femur length) | 62 mm |
| Scan Date | 1 Oct 2026 |
| Gestational age by ultrasound | 32 weeks 0 days |
| Method | Hadlock composite (BPD, HC, AC, FL) |
| Due date from this scan | Thu, 26 Nov 2026 |
| Accuracy at this stage | ±2–3 weeks |
32 weeks 0 days.
Try this example →| FL (femur length) | 33 mm |
| Scan Date | 1 Oct 2026 |
| Gestational age by ultrasound | 20 weeks 2 days |
| Method | average of 1 Hadlock measurement |
| Due date from this scan | Tue, 16 Feb 2027 |
| Accuracy at this stage | ±7–10 days |
20 weeks 2 days.
Try this example →Usually that the baby is a little larger than average, or that you ovulated earlier. Small differences are normal.
HC and BPD in the second trimester; femur length is useful when the head shape is unusual.
Rarely — only if there was no earlier scan and the difference is more than 3 weeks.