Pregnancy Due Date Calculator

When will you meet your baby?

One of the very first questions after a positive test is also one of the most exciting: when is the baby due? Your due date becomes the anchor for everything that follows — every scan, every milestone, every countdown — so getting it right matters from day one.

The standard method is beautifully simple and nearly two centuries old. Naegele’s rule takes the first day of your last menstrual period, adds 280 days (40 weeks), and calls that your estimated due date. Pregnancy is dated from the last period — not from conception — because ovulation day is rarely known with certainty, while period dates usually are.

Enter the first day of your last period below (and your cycle length if you know it), and the calculator will give you your estimated due date, how far along you are right now, and your current trimester.

Naegele’s rule: your due date is about 280 days (40 weeks) after the first day of your last period.

From conception, pregnancy lasts about 266 days (38 weeks).

The three trimesters at a glance

Forty weeks, three acts. Here is what each trimester typically brings — every pregnancy is different, but the broad arc is remarkably consistent.

Trimester Weeks What is happening
First 1 – 12 The embryo becomes a foetus; organs form. Fatigue, nausea, and tender breasts are common. First scan around week 12.
Second 13 – 27 The “golden” trimester — energy often returns. The bump shows, movements begin, and the anatomy scan happens around week 20.
Third 28 – 40 Rapid growth and final preparations. Backache, breathlessness, and frequent bathroom trips as the baby settles head-down.

How due dates are really calculated

Naegele’s rule assumes a 28-day cycle with ovulation on day 14 — tidy, but not everyone’s reality. If your cycles run longer or shorter, ovulation shifts, and so does the true due date; the calculator above adjusts for your cycle length automatically.

Your dating scan (usually between weeks 11 and 14) will refine the estimate further by measuring the baby directly — and scan dating beats period dating when the two disagree. Even then, remember the humbling statistic: only about 5% of babies arrive on their due date. A healthy pregnancy can run from 37 to 42 weeks, so treat the date as the middle of a month-long window, not an appointment.

What to do once you know your date

  1. Book your first antenatal appointment — ideally before week 10, so screening timelines stay on track.
  2. Start folic acid immediately if you have not already (400 micrograms daily through week 12).
  3. Review medications, alcohol, and smoking with your doctor or midwife — some everyday medicines need swapping in pregnancy.
  4. Tell work when you are ready — you are generally required to notify your employer by around week 25, but many women share the news after the 12-week scan.

This calculator is for general information only and is not medical advice. Please confirm your pregnancy and due date with your doctor or midwife, and follow their antenatal care guidance.

Frequently asked questions

Why is pregnancy counted from my last period, not conception?
Because the last period is a date most women know, while the exact day of conception rarely is. Dating from the last menstrual period adds roughly two pre-ovulation weeks — so at a “4 weeks pregnant” positive test, the embryo is actually about two weeks old. Everyone in medicine uses the same convention, so scans, milestones, and advice all line up.
How accurate are due dates?
Reasonably good as estimates, poor as predictions: only around 5% of babies arrive on the exact day. First-trimester ultrasound dating is the most accurate method, usually within a few days. Think of your due date as the centre of a four-week window (37–42 weeks) rather than a deadline.
My cycles are irregular — can I still calculate a due date?
The calculator will give you a rough estimate, but irregular cycles make period-based dating unreliable. Your dating scan becomes the authority here — first-trimester ultrasound measures the baby directly and usually settles the question within days. Mention the irregularity at your first appointment.
What does it mean if I measure ahead or behind at a scan?
Small differences — a few days — are completely normal and usually just reflect natural variation in growth. Larger or persistent discrepancies may prompt your provider to adjust the due date or investigate further. One measurement is a snapshot; the pattern across scans is what matters.
Can the baby come early? When should I worry?
Babies born from 37 weeks are considered full term, and anything before 37 is preterm. Regular painful contractions, waters breaking, or reduced movements at any stage warrant an immediate call to your maternity unit — do not wait to see if it passes.
What happens if I go past my due date?
Very common — around 40% of first-time mothers go past 40 weeks. Your provider will usually offer extra monitoring after 41 weeks and discuss induction around 41–42 weeks, since the placenta becomes less efficient with time. Going “overdue” is not an emergency by itself, but it does deserve close follow-up.