gestational weeks different from ultrasound

Gestational Weeks from LMP Differ from Ultrasound?

A few days' difference between the LMP calculation and ultrasound is normal. Larger differences indicate the EDD should be revised based on fetal biometry.

Medical notice: this calculation is educational, based on population-level averages, and does not replace prenatal care, ultrasound, diagnosis or guidance from a doctor or obstetrician.

Last menstrual period (LMP)First day

Gestational weeks

4weeks

and 0 day(s)

1st trimester

Gestational progress10%

DUM40 weeks

Estimated due date (EDD)Jan 17, 2027

Pregnancy milestones

Wk 4β-hCG high enough for a positive test; primitive heart begins beating; implantation complete
Wk 5Neural tube closing; yolk sac visible; neural plate formed
Wk 6Heartbeat detectable by transvaginal ultrasound; limb buds visible; CRL ~4 mm
Wk 7Distinct brain divisions (forebrain, midbrain, hindbrain); hand plates visible
Wk 8All major organs forming; embryo is now called a fetus; lips and nose take shape
Wk 9External ears developing; eyelids covering eyes; neck visible; CRL ~22 mm
Wk 10Fingers and toes separated; external genitalia beginning to differentiate; fetus can swallow

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When does ultrasound take precedence?

ACOG recommends revising the EDD when: a 9–13-week ultrasound differs by more than 5 days from the LMP-based EDD; or when irregular cycles, PCOS or an uncertain LMP make the calculation unreliable.
After the first trimester, ultrasound precision for dating decreases, as fetal growth variability increases.

Divergence examples

Small difference (within margin)

Input: DUM: 10 semanas e 2 dias · Ultrassom: 10 semanas e 5 dias
Expected output: Diferença: 3 dias — DPP original é mantida

Within the 5-day margin; no revision needed.

Relevant difference (outside margin)

Input: DUM: 10 semanas · Ultrassom: 11 semanas e 2 dias
Expected output: Diferença: 9 dias — DPP deve ser revisada pelo obstetra

The obstetrician decides which EDD to use based on the patient's full history.

Full tool FAQ

Why does the gestational count start before conception?

The gestational count starts at the LMP by universal medical convention, because the exact date of ovulation and fertilization is rarely known precisely. This means that in the first two gestational weeks, pregnancy has not yet occurred biologically — ovulation and fertilization typically happen in weeks 2–3.

What is the difference between gestational age and fetal age?

How many weeks does each trimester have?

Is this calculator accurate for IVF pregnancies?

What should I do if the ultrasound result differs from the calculation?

Why do some months have more weeks than others?

Does my data get saved anywhere?

Does this calculator replace prenatal care?

Frequently asked questions

Is the ultrasound always right?

A first-trimester ultrasound is very accurate but has a margin of a few days. The final decision is the obstetrician's, considering all available data.