Gestation Time by Ultrasound

Pregnancy Dating by Ultrasound

Most of the time gestational age starts from the last menstrual period (LMP) and Naegele’s rule: due date = LMP + 280 days, which is the same as LMP + 9 months + 7 days. If you don’t know the LMP, or it’s shaky, the first-trimester ultrasound takes over as the reference point. Say a scan report reads W weeks + D days on date T. Today’s gestational age is then (W × 7 + D) + (today − T) days.

The Brazilian Ministry of Health prenatal protocol lays out a handful of key scans. At 6–8 weeks you confirm a viable intrauterine pregnancy and a heartbeat. The 11–14 weeks window covers nuchal translucency and aneuploidy screening. Around 20–24 weeks comes the detailed morphological scan. And 28–32 weeks looks at fetal growth, plus 3D/4D imaging if the obstetrician asks for it.

Applications

Getting the dating right is what sets up the rest of prenatal care. It tells you when maternal serum screening should happen, when to run the glucose tolerance test (24–28 weeks), when to culture for Group B Streptococcus (35–37 weeks), and how to think about inducing labor at ≥41 weeks. A scan done before 14 weeks is usually within about ±5 days; do it later and the error can stretch to ±2–3 weeks.

FAQ

Which date wins, LMP or ultrasound? When a first-trimester scan disagrees with the LMP by more than 5–7 days, doctors go with the scan. Talk it through with your obstetrician anyway.

Does this replace a medical exam? No. Treat the number as a rough estimate to bring into your prenatal care. Clinically, only an obstetrician can confirm gestational age.

What if I miscounted the days? A small slip in the inputs moves the estimated due date too. Check it against your prenatal records.