Why count from the last period rather than conception?

Because the date of the last period is knowable and the date of conception generally is not. The convention costs about two weeks of apparent accuracy and buys a starting point that can actually be reported reliably.

How likely is the baby to arrive on the date?

Not very — roughly one in twenty. Anywhere from 37 to 42 weeks is term, a thirty-five-day window, and the due date simply sits in the middle of it.

Due Date Calculator

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Guide to Due Date

How to calculate due date?

The due date is calculated using Naegele's rule, which assumes pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period. This pattern assumes a regular 28-day cycle and ovulation exactly at mid-cycle (day 14). Most babies are born between 37 and 42 weeks of pregnancy, meaning the due date is only approximate - only about 5% of babies are born exactly on their due date. Remember that the due date may vary depending on your menstrual cycle length.

Pregnancy stages

First trimester (1-12 weeks) - period of intense organ development, highest miscarriage risk, typical symptoms include nausea and fatigue
Second trimester (13-26 weeks) - "honeymoon" phase of pregnancy, more energy, visible belly, can learn baby's sex
Third trimester (27-40 weeks) - preparation for labor, baby gains weight, Braxton Hicks contractions may begin

When to contact doctor?

Regular prenatal visits are crucial for maternal and fetal health. If concerning symptoms occur such as severe abdominal pain, bleeding, sudden decrease in fetal movements, intense headaches, or swelling, contact your doctor immediately. By 36 weeks of pregnancy, visits should be weekly. It's also wise to pack your hospital bag in advance in case of early labor.

Preparing for labor

In the final weeks of pregnancy, focus on preparing your body for labor. Regular moderate activity (walking, prenatal yoga) helps prepare muscles. Relaxation and breathing techniques help manage stress. Learn about labor symptoms (Braxton Hicks, baby "dropping", mucus plug) and discuss your birth plan with your doctor. Remember, every pregnancy is different - there's no single "right" way to prepare.

Your due date is measured from two weeks before you were pregnant

Pregnancy is dated from the first day of the last period, not from conception. Since ovulation falls around day 14, the standard forty weeks is really thirty-eight weeks of gestation plus a fortnight of bookkeeping. That convention is why the arithmetic feels off by two weeks — and why the resulting date is a midpoint rather than a prediction.

How it works

Applies Naegele's rule: 280 days from the first day of the last menstrual period.
Adjusts for cycles that are not 28 days, since the rule assumes ovulation on day 14.
Marks the full term window — 37 to 42 weeks — which is what actually matters clinically.

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

if your cycle is not 28 days:
  add (cycle length − 28) days

gestation from conception is 266 days (38 weeks)
the extra 14 days are counted before conception occurred

Worked example

A last period beginning 1 January, with a regular 28-day cycle.

1 January + 280 days = 8 October
that is 40 weeks from the last period
but only 38 weeks from conception around 15 January
early term begins at 37 weeks: 17 September
post-term begins at 42 weeks: 22 October

The due date is 8 October, but the normal arrival window runs 17 September to 22 October — thirty-five days wide. A single date makes a five-week range look like an appointment.

Reading the result

Only about one birth in twenty happens on the due date itself. Treating it as a deadline rather than the centre of a distribution is the source of a great deal of unnecessary anxiety in the final fortnight.
The two-week offset has a practical reason: the last period is a date people can report accurately, while conception usually is not. Precision is traded for reliability.
Naegele's rule assumes a 28-day cycle with ovulation on day 14. On a 32-day cycle, ovulation is later and the date shifts four days out — worth correcting, because uncorrected it can push you toward being labelled post-term early.
A first-trimester ultrasound dates a pregnancy more accurately than the last period and will normally supersede this calculation. Use the arithmetic as an estimate until a scan replaces it.

Common questions

Why count from the last period rather than conception?: Because the date of the last period is knowable and the date of conception generally is not. The convention costs about two weeks of apparent accuracy and buys a starting point that can actually be reported reliably.
How likely is the baby to arrive on the date?: Not very — roughly one in twenty. Anywhere from 37 to 42 weeks is term, a thirty-five-day window, and the due date simply sits in the middle of it.

Sources

ACOG — Methods for Estimating the Due Date