The standard estimate is Naegele's rule: due date = last period + 280 days (40 weeks) — and this tool also works from a known conception date (+266 days) or an IVF embryo transfer (+263 for a day-3 embryo, +261 for day-5). Enter your dates below for your due date, current week, trimester, and milestones — but read it like a clinician does: only about 4–5% of babies arrive on the exact day.
Pregnancy dating counts from the first day of the last menstrual period — a convention older than ultrasound, because the LMP is a date most people can identify while conception usually is not. The rule: due date = LMP + 280 days, assuming ovulation on day 14 of a 28-day cycle.
Two standard adjustments: with a non-28-day cycle, shift the date by (cycle length − 28) days — later ovulation means a later due date. And from a known conception date instead, use conception + 266 days, which lands on the same date when the 28-day assumption holds.
Not very, and clinicians know it: only about 4–5% of babies are born on their exact due date. Most arrive within roughly two weeks either side, which is why the date works as a planning anchor — for scheduling scans, leave, and preparation — rather than a deadline. If a first-trimester ultrasound disagrees with the LMP estimate beyond a few days, the ultrasound wins: crown-rump length measurement is the most accurate dating method, accurate to about ±5–7 days (ACOG Committee Opinion No. 700). This is also the dependable route when cycles are irregular — LMP dating assumes a 28-day cycle with day-14 ovulation, and a varying cycle breaks that assumption.
| Trimester | Weeks | Notes |
|---|---|---|
| First | Week 1 – 13+6 | Organ formation; dating ultrasound is most accurate in this window |
| Second | Week 14 – 27+6 | Anatomy scan typically at 18–22 weeks |
| Third | Week 28 – birth | Full term from week 39; due date at week 40 |
Working backwards instead? The conception calculator estimates when conception likely happened. Not yet sure you're pregnant? Start with the pregnancy test calculator.
Naegele's rule: last period + 280 days. With a non-28-day cycle, shift by (cycle length − 28) days.
Only about 4–5% of babies arrive on the exact date; most come within roughly two weeks either side. It's a planning anchor, not a deadline.
Adjust by (cycle length − 28): a 35-day cycle moves the due date about 7 days later. The calculator applies this automatically.
Yes — a first-trimester ultrasound (crown-rump length, accurate to about ±5–7 days) overrides the LMP estimate when they disagree. ACOG’s rule of thumb: the earlier the scan, the smaller the gap needed to redate — about >5 days before 9 weeks, >7 days at 9–13+6 weeks, wider later in pregnancy.
Pregnancy counted from the last period, so it runs about 2 weeks ahead of the time since conception: "8 weeks pregnant" ≈ 6 weeks since conception.
Conception + 266 days — equivalent to LMP + 280 when the 28-day assumption holds.
First: through 13+6. Second: 14–27+6. Third: from 28. Full term from week 39.
LMP dating becomes unreliable because the ovulation assumption breaks — a first-trimester ultrasound is the dependable way to date the pregnancy.
Digital tests with a weeks indicator (like Clearblue) count from conception, while clinicians count from the last period — about 2 weeks earlier. So a “1–2” reading means roughly 3–4 weeks pregnant, “2–3” means 4–5 weeks. Enter your last period above to see the clinical week.
IVF dating is more precise because the embryo’s age is known exactly: due date = transfer day + (266 − embryo age in days). A day-5 blastocyst transfer means +261 days; a day-3 transfer means +263 days. The transfer date replaces the LMP entirely — no cycle-length adjustment applies.
Apps assume ovulation on day 14 of a 28-day cycle; your doctor has your actual scan. When a first-trimester ultrasound and the LMP estimate disagree beyond a few days, ACOG says the ultrasound wins. Treat the app date as the placeholder until the dating scan.
LMP-based dating needs a date you can identify. If you can’t, skip the estimate and rely on a first-trimester ultrasound — crown-rump length dating is accurate to about ±5–7 days and doesn’t depend on memory. Testing first? The pregnancy test calculator covers timing.
Pregnancy is 40 weeks but only about 9 calendar months + 1 week — months aren’t 4 weeks, so the two never line up cleanly. Rough mapping: 4 weeks = month 1, 8 weeks = month 2 … 40 weeks = end of month 9. Clinicians use weeks precisely because months are ambiguous.