The estimated due date, how many weeks of pregnancy have elapsed, and which trimester that falls in — with the cycle length taken into account.
These dates are estimates from a calendar rule, not medical advice. Your midwife or doctor has the final word.
The day bleeding started, not the day it ended.
From the first day of one period to the first day of the next.
Estimated due date
Gestational age
Trimester
Estimated conception
What this does not tell you
The estimated due date comes from Naegele’s rule: 280 days — 40 weeks — from the first day of the last period. It has been the standard since 1812 and it carries an assumption worth stating, because most calculators do not: it assumes a 28-day cycle. In a 35-day cycle ovulation arrives about a week later and so does the birth, so applying the raw 280 days there produces a date a week early. This calculator shifts the date by the difference between your cycle and 28 days.
The estimated date is the middle of a distribution, not an appointment. Fewer than 5% of births happen on it; most arrive within the two weeks either side. A first-trimester ultrasound dates a pregnancy more accurately than any calculation from the last period, and it is what obstetric care uses — this page states the method it applied and the margin that method has, rather than presenting a date as a fact.
Gestational age is counted from the first day of the last period, which means the count starts about two weeks before conception. That is why someone can be "four weeks pregnant" two weeks after conception, and it is the convention every clinic uses. It is written as weeks and days — 12w3d is twelve completed weeks and three days. The trimesters divide that span: the first runs to the end of week 13, the second to the end of week 27, the third from week 28 onwards.
Naegele’s rule adds 280 days to the first day of the last period. Because the rule assumes a 28-day cycle, this calculator adjusts by the difference: a 32-day cycle moves the estimated date four days later, a 24-day cycle four days earlier.
Fewer than 5% of births occur on it. It marks the centre of a range rather than a scheduled day, and a first-trimester ultrasound dates a pregnancy more precisely than any calculation from the last period — which is why obstetric care revises the date once a scan is available.
Because the first day of the last period is a date people can identify, and the day of conception usually is not. The convention adds roughly two weeks at the front, so gestational age is about two weeks ahead of the time since conception. Every clinical figure — the 40 weeks, the trimester boundaries, the scan schedule — is stated on that same basis.
They are conventional divisions of the 40 weeks, counted from the last period: the first trimester through the end of week 13, the second through the end of week 27, the third from week 28 to birth. The boundaries are a way of describing where a pregnancy is, not events that happen on a particular day.
Twelve completed weeks of gestation plus three days — the notation used in maternity records. It is counted from the first day of the last period, so it is a position in the 40-week span rather than the time elapsed since conception.
Body mass index from your height and weight, with the range it falls in.
Estimated daily energy use from the Mifflin-St Jeor equation, with its margin.
Estimated fertile window and ovulation day from your cycle length.
Add, subtract, multiply and divide fractions — exactly, never rounded.
Solve a proportion — with the inverse case the usual formula gets wrong.
What is X% of Y — the everyday percentage, worked out step by step.