Pregnancy Due Date Calculator: How to Calculate Your Due Date

A due date sounds like a deadline, but it is really a midpoint estimate. Only about 4% of babies actually arrive on their predicted date; the rest land within a 5-week window around it. Still, the calculation is one of the first things a pregnancy provider does — every scan, blood test and milestone is timed off it.

How due dates are calculated

The most common formula is Naegele's rule, developed in the 1800s and still standard:

Due date = first day of last menstrual period + 280 days (40 weeks)

Equivalent shortcut: subtract 3 months from your last period and add 7 days.

Example. If your LMP began on 15 January 2026, due date = 22 October 2026.

Naegele assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the date shifts by the same number of days.

Ultrasound vs LMP dating

First-trimester ultrasound (6–13 weeks) is more accurate than LMP for due-date estimation — it measures the embryo's crown-rump length directly, with ±5–7 day accuracy. If LMP and ultrasound disagree by more than a week, providers usually go with the ultrasound.

By the third trimester, ultrasound dating is much less reliable (±3 weeks), because babies grow at different rates.

The three trimesters

First trimester (weeks 1–13): - Major organ formation - Highest miscarriage risk (drops sharply after week 12) - Common symptoms: fatigue, nausea, breast tenderness

Second trimester (weeks 14–27): - Often the most comfortable - Anatomy scan around 20 weeks - Movement felt around 18–22 weeks

Third trimester (weeks 28–40): - Rapid weight gain (~0.5 kg/week) - Glucose screening for gestational diabetes - Group B strep test around 36 weeks - Pre-eclampsia and blood-pressure monitoring

What to expect, week by week (highlights)

• Week 6 — heartbeat detectable on ultrasound
• Week 12 — second trimester begins; risk of miscarriage falls
• Week 16 — fingerprints form
• Week 20 — anatomy scan; baby ~half a kg
• Week 24 — viability threshold in modern NICUs
• Week 28 — third trimester; eyes open
• Week 37 — full-term begins
• Week 40 — official due date
• Weeks 41–42 — post-term; induction usually discussed

Common dating questions

What if I don't know my LMP? Early ultrasound (8–12 weeks) is the standard substitute.

Twins or higher-order multiples? Same calculation, but they typically deliver 2–4 weeks earlier and the due date is sometimes adjusted accordingly.

IVF pregnancies? Use the egg-retrieval or embryo-transfer date — far more accurate than LMP.

Healthy lifestyle reminders

• Folate (400 mcg/day) ideally starting pre-conception, continued through first trimester.
• No alcohol, no smoking. Both raise miscarriage and developmental risk.
• Limit caffeine to under 200 mg/day.
• Avoid raw fish/meat, soft cheeses, deli meats unless heated.
• Stay active — moderate exercise reduces gestational diabetes and back pain. Aim for 150 min/week.
• Sleep on your left side in the third trimester to maximise blood flow.

Frequently asked questions

Can my due date change? Yes — most often after the first ultrasound. After 14 weeks, providers rarely move it.

How accurate is a calculated due date? ±5 days for ultrasound, ±10 days for LMP-only.

Should I plan for the due date exactly? No. Plan for the 38–42 week window. Arrange parental leave, hospital bag and birth partner availability accordingly.

Try our Pregnancy Due Date Calculator — it accepts LMP, ultrasound or IVF dates and gives a personalised trimester timeline.