Pregnancy Due Date Calculator — LMP Method
The due date every clinic quotes starts from one date and one rule: the first day of the last menstrual period plus 280 days — Naegele’s method, the convention obstetrics has used for two centuries. This tool computes the EDD, today’s gestational age in weeks and days, and the trimester — with the honest caveat that the date is a planning estimate, not an appointment.
How accurate is the LMP due date?
It’s a distribution centre, not a prediction: only a small fraction of births land on the EDD itself, while the large majority arrive within the two weeks either side. The method also assumes a regular 28-day cycle with ovulation mid-cycle — irregular cycles shift the true date, which is why the first-trimester ultrasound’s dating usually supersedes the LMP estimate when they differ.
Why do doctors count pregnancy from the LMP and not conception?
Because the LMP is a date women can actually report, while conception is rarely known precisely. The convention makes "weeks pregnant" run about two weeks ahead of embryonic age — week 6 by LMP is roughly four weeks post-conception — which surprises many couples reading the numbers for the first time.
My cycles are irregular — should I trust this date?
Treat it as a rough opening estimate only. Irregular or long cycles displace ovulation and the LMP arithmetic with it; the dating ultrasound between roughly 8 and 13 weeks measures the embryo directly and produces the EDD your antenatal care should run on. Bring both dates to the booking visit and let the scan arbitrate.
Pregnancy Due Date Calculator
What the 280 days are actually counting
Naegele’s rule adds 280 days — 40 weeks — to the LMP’s first day, bundling the roughly two pre-ovulation weeks into the count by convention. The gestational-age clock that results runs all antenatal medicine: screening windows, scan schedules, medication decisions, and viability thresholds all speak in LMP-weeks. The practical habit for expectant parents is recording the LMP date the moment pregnancy is suspected — it is the one data point the whole nine months keeps asking for.
The scan that resets the clock
First-trimester ultrasound measures crown-rump length against growth curves tight enough to date the pregnancy within days — which is why a scan EDD differing from the LMP one by more than about a week replaces it, permanently, in competent care. Later scans date progressively worse as babies diverge in size, so the third-trimester "the baby measures two weeks ahead" line describes growth, not a new due date. One dating, done early, then left alone: that is the obstetric standard worth insisting on.
Using the date well in the Pakistani care context
The EDD’s real function is scheduling backwards: booking visit in the first trimester, anomaly scan in the 18–22 week window, glucose screening where indicated around 24–28, and the delivery-planning conversations — facility choice, who attends, the transport plan — settled by the start of the third trimester rather than during the first contractions. Where distance to facilities is real, the plan matters more than the date’s precision. This page computes; the antenatal clinic cares — book the visit.
More questions answered
The booking visit belongs in the first trimester — ideally by week 10–12 — covering history, baseline blood pressure, blood and urine tests, and the dating scan. Early booking is the single intervention most associated with catching the manageable problems (anaemia, thyroid, blood-group issues) while they’re still manageable; the pattern of first arriving at a clinic in month seven gives those problems a head start.
Convention more than biology, but useful convention: the first trimester (to ~13 weeks) carries the organ-formation period and most miscarriage risk; the second (to ~27) is typically the easiest physically and hosts the anomaly scan around weeks 18–22; the third is growth, position, and delivery preparation. Care schedules and many precautions key off the boundaries.
The standard urgent list: bleeding, severe abdominal pain, severe headache or visual disturbance, significant swelling of face and hands, reduced fetal movement after it has established, fever, and fluid leakage. None of these waits for the scheduled appointment — emergency assessment exists for exactly this list.
Term runs as a window around the EDD: early term from 37 weeks, full term 39–40, late term 41, post-term past 42 with its own monitoring decisions. The EDD anchors the window; deliveries anywhere inside 37–42 weeks are within the normal range the date was always pointing at.
IVF pregnancies date precisely from the transfer (embryo age plus transfer date), and clinics compute that directly — more accurate than any LMP arithmetic. For a known conception date, adding 266 days approximates the same EDD this tool reaches via LMP-plus-280; the IVF clinic’s date, where one exists, governs.