How is gestational age calculated from LMP?

Gestational age (GA) = today minus the first day of your last menstrual period, counted in days, then expressed as complete weeks plus remainder days. Using Naegele's rule, the estimated due date (EDD) = LMP + 280 days (40 weeks). So if your LMP was 70 days ago, you are 10 weeks + 0 days pregnant.

What is Naegele's rule?

Naegele's rule is the standard obstetric formula for estimating a due date: add 280 days (40 weeks) to the first day of the last menstrual period. It assumes a regular 28-day cycle and ovulation on day 14. The rule has been used clinically since the 19th century and remains the default when no ultrasound dating is available.

What is the difference between gestational age and fetal age?

Gestational age is counted from the LMP and is the standard clinical measure — it is roughly 2 weeks longer than fetal age (which is counted from conception). When a doctor says you are 12 weeks pregnant, they mean 12 weeks of gestational age, not 12 weeks since the embryo formed.

When does each trimester start and end?

The first trimester covers weeks 1–13 (days 0–97 from LMP), the second trimester covers weeks 14–27 (days 98–195), and the third trimester runs from week 28 to delivery (days 196–280+). These boundaries follow standard clinical convention; some sources place the second-trimester start at week 13+1 rather than week 14.

My EDD from a scan is different from this calculator — which is right?

The ultrasound estimate is more accurate, particularly if it was performed in the first trimester (before 14 weeks). Naegele's rule assumes a perfect 28-day cycle with ovulation on day 14 — any variation shifts the true conception date. Always defer to the date your clinician assigns after a dating scan.

Is this calculator private?

Yes. All dates and calculations happen entirely inside your browser using JavaScript. Nothing is sent to a server, stored, or logged. You can use it offline once the page has loaded.

Pregnancy Week Calculator

The pregnancy week calculator tells you exactly how many weeks and days pregnant you are, which trimester you are in, when your estimated due date (EDD) falls, and which developmental milestones you have already passed — all from a single date entry. You can enter either the first day of your last menstrual period (LMP) or an EDD you already have from a scan.

How it works

The tool uses Naegele’s rule, the internationally accepted obstetric formula for estimating gestational age and due date. The two key equations are:

EDD = LMP + 280 days Gestational age = today - LMP (expressed as weeks + remaining days)

For example, if your LMP was 1 January 2026, then:

EDD = 1 January 2026 + 280 days = 8 October 2026
On 1 April 2026 (90 days later), gestational age = 12 weeks + 6 days
Trimester: first (days 0–97)

If you enter an EDD instead, the calculator reverses the formula — LMP is derived as EDD minus 280 days — and then applies the same gestational-age arithmetic.

Trimester boundaries follow standard clinical convention:

Trimester	Gestational weeks	Days from LMP
First	1 – 13	0 – 97
Second	14 – 27	98 – 195
Third	28 – 40+	196 – 280+

Worked example

Suppose your LMP was 15 February 2026 and today is 1 June 2026 (105 days later):

Gestational age: 105 ÷ 7 = 15 weeks + 0 days
Trimester: second (day 105 falls in the 98–195 range)
EDD: 15 February + 280 days = 23 November 2026
Days remaining: 23 November minus 1 June = 175 days
Percentage complete: (105 ÷ 280) x 100 = 37.5%
Next milestone: Week 16 — gender may be visible on ultrasound; skeleton hardening

The progress bar fills proportionally from 0% at the LMP to 100% at the EDD, giving you an at-a-glance sense of how far along the pregnancy is.

Understanding gestational age vs fetal age

Gestational age starts counting from the LMP, roughly two weeks before conception, because ovulation occurs approximately on day 14 of a standard cycle. This means a fetus that is clinically described as “12 weeks” has actually been developing for about 10 weeks since fertilisation. Gestational age is universally used by clinicians because the LMP date is known, whereas the exact date of conception usually is not.

The calculator reports gestational age throughout — consistent with what your midwife or obstetrician will use. If you see a different number on your scan report, confirm with your clinician which dating method was applied, as early ultrasound measurements can shift the assigned EDD by several days.

Educational use only — not medical advice. This tool provides an estimate based on standard Naegele’s rule. Always confirm your due date and gestational age with a qualified healthcare professional. Ultrasound dating, especially before 14 weeks, is more accurate than LMP-based calculation.