Pregnancy Weeks to Months

What the calculator does

Enter a term in weeks — or pick a month of pregnancy — and the page says how many obstetric and calendar months that is, which trimester it falls into, how many weeks are left to the full term and when the estimated due date is.

An obstetric month is four weeks, and that is the whole confusion

An obstetric month is exactly 4 weeks, or 28 days. A calendar month averages 30.4375 days. The two counts drift apart from the first month, and by the end of a pregnancy they differ by about 0.8 of a month:

That is why a doctor says «ten months» and everyone else says «nine», and both are right. Every page of this calculator prints both numbers, because the question is almost always about the disagreement rather than about either number alone.

Where the count starts

Pregnancy is dated in completed weeks from the first day of the last menstrual period, not from conception. That is the convention the count is built on, and the calculator does nothing else with it: the two starting points are about two weeks apart, so a term counted from one of them does not match a term counted from the other.

Trimesters

• First trimester — weeks 1 to 13.
• Second trimester — weeks 14 to 27.
• Third trimester — week 28 until birth.

The boundaries are the ones used by ACOG. They fall inside obstetric months rather than on their edges, which is why a month of pregnancy can belong to two trimesters at once — the calculator says so when it happens.

How the due date is calculated

This is Naegele's rule. The calculator works backwards from the term you entered: if the term is right today, the remaining part of those 280 days lands on the estimated date. Past 40 weeks the same arithmetic puts that date in the past, and the page prints it as such, saying plainly that the full term is already behind — beyond term what happens next is a clinical question, not an arithmetic one.

What this calculator is not

It is arithmetic, not medical advice. The date it prints is an estimate produced by one formula, and a real birth can fall on either side of it. It says nothing about screenings, ultrasounds, maternity leave or any other question that depends on a doctor or on the law where you live.