Pregnancy Due Date Calculator

How the estimate is made

The standard method is Naegele's rule: add 280 days, 40 weeks, to the first day of your last menstrual period.

That works because pregnancy is conventionally dated from the last period rather than from conception, which is rarely known precisely. The consequence is counterintuitive: the first two weeks of a "40-week pregnancy" happen before conception. You are considered four weeks pregnant roughly two weeks after conception.

The rule assumes a 28-day cycle with ovulation on day 14. Cycles vary, so the tool adjusts using the more accurate assumption that ovulation occurs about 14 days before the next period. On a 35-day cycle, that shifts the due date about a week later.

What "due date" actually means

It is the midpoint of a distribution, not an appointment.

• About 4% of babies are born on their due date.
• Roughly 80% arrive between 37 and 42 weeks.
• Around 60% of first-time mothers go past their due date.
• Term is anything from 37 to 42 weeks; before 37 is preterm, after 42 is post-term.

A first-trimester ultrasound dates a pregnancy to about ±5 days, and is the figure your care team will use if it differs meaningfully from your period-based date. Later scans are progressively less accurate for dating, because babies grow at different rates.

The trimesters

Trimester	Weeks	Broadly
First	0–13	Organs form. Fatigue and nausea are common. Highest miscarriage risk, falling sharply after a heartbeat is seen.
Second	14–27	Often the most comfortable stretch. Movement becomes noticeable around 18–22 weeks. Anomaly scan usually around 20 weeks.
Third	28–40+	Rapid growth. Increasing discomfort and disturbed sleep. Viability improves substantially each week.

Dating an IVF pregnancy

IVF dating is more precise, because the date of fertilisation is known exactly. The due date is calculated from the transfer date, adjusted for the embryo's age:

• Day 3 transfer: transfer date + 263 days
• Day 5 (blastocyst): transfer date + 261 days
• Day 6 transfer: transfer date + 260 days

Your clinic will give you a date; use theirs.

Roughly when things happen

Care varies considerably by country, but a common pattern:

• 8–12 weeks: booking appointment, dating scan, initial bloods.
• 11–14 weeks: combined screening where offered.
• 18–21 weeks: anomaly scan.
• 24–28 weeks: glucose screening in many systems.
• 28 weeks onward: appointments become more frequent.
• 36 weeks: position checked, birth plan discussed.
• 40–42 weeks: monitoring, and a conversation about induction if you go past term.

This is arithmetic, not medical care. It cannot detect anything, and it does not replace antenatal appointments, scans or screening. Register with a midwife or doctor as early as you can, and always use the date your care team gives you over the one calculated here. Seek urgent medical advice for bleeding, severe pain, reduced fetal movement, or any concern at all. Do not wait for a scheduled appointment.