Your estimated due date, current week, and trimester.

How is the due date calculated?

By default this uses Naegele's rule: the first day of your last menstrual period (LMP) plus 280 days (40 weeks), adjusted for a cycle length other than 28 days. The conception-date and IVF transfer-date methods convert to an equivalent LMP date first (conception ≈ LMP + 14 days; a day-5 blastocyst transfer ≈ LMP + 19 days), then apply the same 280-day rule.

Why does pregnancy get counted from before conception?

Medical convention counts gestational age from the first day of the last menstrual period, not from conception (which happens roughly two weeks later) — so you're considered a couple of weeks "pregnant" before conception technically occurs. This is standard obstetric practice, not an error in this calculator.

How accurate is this due date?

Only about 5% of babies arrive exactly on the estimated due date — it's a statistical midpoint of a normal range (most births happen within about 2 weeks either side), not a firm prediction. A first-trimester dating ultrasound is generally considered more accurate than a last-period calculation alone, especially if your cycles are irregular.

What are the trimester boundaries?

First trimester: weeks 1–13. Second trimester: weeks 14–26. Third trimester: week 27 onward until birth around week 40. "Full term" begins at week 37, and week 24 is commonly cited as a viability milestone in medical literature.

Why pregnancy is dated from your last period

Standard dating uses Naegele's rule: 280 days from the first day of your last menstrual period. That is 40 weeks — but it counts from roughly two weeks before conception actually occurred, because the LMP date is knowable and the conception date usually isn't.

The consequence catches almost everyone: at four weeks pregnant, the embryo is about two weeks old. "Gestational age" and the actual age of the pregnancy differ by around a fortnight throughout.

The estimate assumes a 28-day cycle

Naegele's rule assumes ovulation on day 14. If your cycles run longer, ovulation is later and the LMP-based due date is correspondingly too early — on a 35-day cycle, by about a week. This is why an early ultrasound frequently revises the date, and the ultrasound is the more reliable of the two.

A first-trimester scan measuring crown-rump length is accurate to within about five days. Later scans are progressively less precise for dating, because babies grow at increasingly different rates.

The trimesters

• First — weeks 1-13. All major organ systems form. Most miscarriages occur here.
• Second — weeks 14-27. Movement becomes noticeable; the anatomy scan usually falls around weeks 18-22.
• Third — weeks 28-40. Rapid growth and lung maturation.

Full term is 39-40 weeks; 37-38 is early term and 41+ is late term. Only about 4% of babies arrive on their due date — the date is the centre of a distribution, not a prediction, and most births fall within two weeks either side.

Use it as a reference point

The due date organises appointments, screening windows and leave planning, and that is its real function. It is not a deadline, and going past it is normal rather than a problem in itself.

Related tools: the Pregnancy Week Calculator for where you are now, the Pregnancy Weight Gain Calculator for the recommended range given your starting BMI, and the Conception Calculator to work backwards. None replaces your midwife or doctor.