Gestational Age Calculator

Estimate gestational age in weeks and days from last menstrual period, due date, or conception date, with trimester and due date.

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First day of last menstrual period

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Health — Pregnancy

Gestational Age Calculator

Gestational age is the standard way of tracking how far along a pregnancy is. Measured in weeks and days from the first day of the last menstrual period (LMP), it underlies all pregnancy milestones, trimester transitions, antenatal appointments, and due date estimates.

About this calculator

Reviewed March 2026

Methodology

Gestational age from LMP: total days = today − LMP; weeks = floor(totalDays / 7). EDD = LMP + 280 days. From conception: LMP equivalent = conception − 14 days. Trimesters: T1 = 0–12 weeks, T2 = 13–26 weeks, T3 = 27+ weeks, consistent with standard obstetric practice.

Limitations

LMP-based dating assumes a 28-day cycle; irregular cycles reduce accuracy.
Early ultrasound scan is the clinical gold standard for gestational age dating.
This calculator does not account for multiple pregnancies or IVF-specific timing.

Disclaimer

Gestational age estimates from this tool are informational only. Clinical dating should be confirmed by a midwife or obstetrician using appropriate investigations.

See our calculation methodology and editorial standards for how this estimate is produced.

How gestational age is calculated

Gestational age is counted from the first day of the last menstrual period — not from conception. Because ovulation and fertilisation occur roughly 14 days after LMP, gestational age is approximately two weeks ahead of conceptional age. From LMP, gestational age in days equals today minus LMP, divided by 7 to give weeks. From a known due date, gestational age is 40 weeks minus weeks remaining until EDD. From conception, add 14 days to the conception date to find the LMP equivalent, then calculate from there.

Trimesters and milestones

The first trimester covers weeks 1–12 and includes the period of major organogenesis. The second trimester covers weeks 13–26 and is typically when the anomaly scan is performed (18–21 weeks). The third trimester covers weeks 27 onwards until delivery. Term is defined as 37–42 weeks; most antenatal care is structured around these gestational age milestones.

Why accurate dating matters

Gestational age is used to schedule antenatal screening such as the nuchal translucency scan (11–14 weeks) and anomaly scan (18–21 weeks), to interpret test results in the correct context, and to plan delivery timing for pregnancies reaching 41–42 weeks. The EDD is LMP plus 280 days. Early ultrasound scanning between 8 and 13 weeks provides the most accurate dating, as foetal size is highly predictable at this stage.

Frequently asked questions

Is gestational age the same as age from conception?

No. Gestational age is counted from the first day of the last menstrual period, which is typically about two weeks before ovulation and fertilisation. A pregnancy described as "12 weeks" is approximately 10 weeks from conception.

Why does gestational age start from the last period?

LMP is a reliable, known date — the day of conception is usually not known precisely. Using LMP as the reference point provides a consistent standard across all pregnancies.

How accurate is gestational age from LMP?

LMP-based dating assumes a regular 28-day cycle with ovulation on day 14. For women with irregular cycles, an early ultrasound scan provides more accurate dating. Most pregnancies are confirmed or adjusted by scan at the 12-week booking appointment.

Gestational Age Calculator

Gestational Age Calculator

How gestational age is calculated

Trimesters and milestones

Why accurate dating matters

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