How many weeks pregnant am I? How dating actually works
Pregnancy dating contains a genuine oddity that confuses almost everyone at their first appointment: the clock starts before the pregnancy does. Clinicians count from the first day of your last menstrual period (LMP) — roughly two weeks before conception typically happens. So on the day you conceive, you're already 'two weeks pregnant' by medical convention. It's not an error; it's simply the one date most people can name with confidence, standardised across obstetrics.
From that anchor, the arithmetic is fixed: a full-term pregnancy is dated at 280 days — 40 weeks — from the LMP, or 266 days from conception. This pregnancy calculator accepts whichever date you know (last period, conception, or an already-given due date) and works out the rest: your current week, trimester, and the milestones ahead.
The three trimesters at a glance
| Trimester | Weeks | What tends to happen |
|---|---|---|
| First | 1–12 | Organs form; dating scan; morning sickness peaks |
| Second | 13–27 | Anatomy scan (~20 wks); movement felt; energy returns |
| Third | 28–40+ | Rapid growth; more frequent checkups; birth |
Only about 4–5% of babies arrive on their exact due date. The honest reading of '40 weeks' is a window: most births happen between weeks 38 and 42.
When the calculator's answer will differ from your doctor's
Two common cases. First, irregular or long cycles: LMP-based dating quietly assumes ovulation on day 14 of a 28-day cycle, so if your cycles run 35 days, standard dating can overstate your progress by a week. The calculator's cycle-length setting corrects for this — use it. Second, ultrasound: an early dating scan measures the embryo directly, and when it disagrees with your LMP by more than a few days, clinicians re-date the pregnancy to match the scan. That re-dated version wins from then on.
Related tools cover the neighbouring questions: the due date calculator focuses on the finish line, the conception calculator works the timeline backward, and the pregnancy weight gain calculator tracks healthy gain by trimester.
Frequently asked questions about pregnancy weeks
How is pregnancy week calculated?
Clinical pregnancy dating usually starts from the first day of your last menstrual period (LMP), not conception. That is why you are "2 weeks pregnant" at conception biologically.
What are the three trimesters?
Trimester 1 is weeks 1–12, trimester 2 is weeks 13–27, and trimester 3 is weeks 28 until birth. Milestones and symptoms vary within each stage.
Can my due date change after ultrasound?
Yes. Early ultrasound can adjust dating if it differs from LMP-based estimates, especially with irregular cycles.
Sources and further reading
- ACOG — Methods for estimating the due date
- NHS — Pregnancy due date information
- MedlinePlus — Pregnancy
This page is for general information only and is not medical advice. Always confirm pregnancy dating and care decisions with your doctor or midwife.