How to Calculate Your Pregnancy Week (Gestational Age)

You've just told the clinic your last period started three months ago. "Congratulations, you're about 13 weeks along," says the midwife. Your partner whispers, "But we only found out six weeks ago… does she think it's someone else's?"

That nudge of confusion at the start of pregnancy is nearly universal, and it has a perfectly logical explanation. Pregnancy weeks are counted from your last menstrual period (LMP), not from conception. Conception happens roughly two weeks after your LMP — which is why you're considered "2 weeks pregnant" at the moment of conception, and why gestational age always runs about two weeks ahead of what your romantic calendar suggests. It's a quirk of history: for centuries, before ultrasound existed, the only hard date a woman could give was the first day of her last period, and the entire convention of counting pregnancy grew from that.

How gestational age is calculated

Gestational age (weeks) = days since LMP ÷ 7

A full-term pregnancy is classically 280 days — 40 weeks — and the whole edifice of due dates and trimesters hangs off that number.

Worked example

LMP was 15 January 2026; today is 14 August 2026:

Days elapsed: 211 days
Weekly age: 211 ÷ 7 = 30 weeks and 1 day

So the gestational age is 30 weeks and 1 day — and since 211 is about 75% of 280, the pregnancy is roughly three quarters complete by the calendar.

Reversing it: guessing dates from a due date

No LMP on record? The reverse is equally standard. If you know your due date, subtract 280 days:

LMP = due date − 280 days

A due date of 22 October 2026 implies an LMP of 15 January 2026 — the same starting point as the example above, confirming the arithmetic hangs together. (The classic bedside version of this is Naegele's rule, named after the German obstetrician Karl Naegele in the early 19th century: take the LMP, add one year, subtract three months, add seven days. LMP October 15 → add a year → October 15 → back three months → July 15 → add seven days → July 22: a due date. The method is older than the country you're reading this from, which tells you how long we've been doing this maths.)

Why the first two weeks feel empty

The most disorienting part of pregnancy dating is that weeks 1 and 2 exist before anything has happened. The "countdown" starts on day one of your period; ovulation and conception land around the end of week 2; and by the time the test turns positive — usually around week 4 or 5 by this counting — you're already "a month pregnant" despite only conceiving a fortnight earlier. There's nothing wrong with your dates. The timeline is just anchored to a different reference point than the one intuition reaches for.

The trimesters, by numbers

Trimester	Weeks	Typical landmarks
First	1–13	Major organ systems form; nausea and fatigue are common
Second	14–27	Energy often returns; movements usually felt; the anatomy scan happens here
Third	28–40	Rapid growth, heavier baby, and birth preparation

Trimesters are convenience buckets rather than sharp biological borders — a week 13 day and a week 14 day are nearly identical, not different eras.

The due date is an estimate, with witnesses

Here's the stat that reconnects the spreadsheet to real life: only a small fraction of babies are born on their due date — the conventional figure is around 4–5% — while a large share arrive within two weeks either side. The 40-week count is a statistical centre line, not a delivery appointment. A "term" pregnancy is generally considered to span roughly 37 to 42 weeks, with care teams watching the boundaries of that window.

When the calendar needs a better witness

LMP dating assumes a regular 28-day cycle, which many people simply don't have. If your cycles are irregular, you don't remember the date, or you conceived with the help of treatment, the LMP arithmetic gets shakier. In those cases — and, really, in every case — a first-trimester ultrasound provides the most accurate dating, measuring the embryo early when growth is extremely consistent from person to person. When the scan's dating and the LMP dating disagree, your doctor's dating wins. Along the way, if symptoms, bleeding, or worry appear, a care provider is the right port of call — the due-date maths is fascinating, but it's not medical advice.

Key takeaways

• Gestational age counts from the first day of your LMP, not from conception — a historical convention that runs ~2 weeks ahead of conception.
• Formula: gestational age = days since LMP ÷ 7; full term = 280 days (40 weeks), per Naegele's rule.
• Trimesters are 1–13, 14–27, and 28–40 weeks, in broadly round numbers.
• Due dates are estimates: few babies arrive exactly on the date, and a first-trimester ultrasound is the most reliable dating method.

FAQ

Why does my app say a different week than the clinic? Apps often count from conception or use cycle-based adjustments, while clinics anchor to the LMP convention, ultrasound dating, or both. Ask which reference point your provider uses, and calibrate your app to it.

My cycle isn't 28 days — do I count differently? The formula still starts from LMP, but the assumed two-week offset between LMP and ovulation grows if your cycles are longer (or shrinks if shorter). That's precisely when ultrasound dating earns its keep.

When does my "second trimester" actually start? The 1–13 / 14–27 / 28–40 split is a widely used convention, but boundaries vary a little between guidelines and countries. The exact week matters far less than the care milestones your provider schedules around it.

Get your exact week and progress with the Pregnancy Week Calculator so the 40-week countdown stays clear from the very first positive test.