Pregnancy Calculator

This calculator puts the whole pregnancy timeline in one place. Enter what you know and it works out your due date, how far along you are today, which trimester you are in, and the dates of the standard milestones along the way.

It is the broadest of the tools here. If you only need one answer, the due date calculator and the pregnancy week calculator are quicker.

These calculators provide estimates for informational purposes only. They are not intended to diagnose, treat, prevent, or replace professional medical advice. Pregnancy and menstrual cycles can vary, so results may not be exact.

How this calculator works

What you need to enter:

The first day of your last menstrual period, or your estimated due date
Your average cycle length, if you are starting from your last period

One timeline, two ways in

Everything on this page is anchored to a single number: the days elapsed since the first day of your last menstrual period. That number is what clinicians call gestational age.

You can supply it either way round. Give a last period date and the calculator adds 280 days to find the due date. Give a due date and it subtracts 280 days to recover the equivalent last period. Both routes produce the same timeline.

How the trimesters are divided

Trimesters are conventions for splitting the 40 weeks into three roughly equal stretches. This site uses the common division: the first trimester runs through the end of week 13, the second covers weeks 14 to 27, and the third begins at week 28.

You will occasionally see slightly different boundaries elsewhere, because the split is a convenience rather than a biological threshold. Nothing changes about the pregnancy on the day a trimester rolls over.

First trimester: 0 weeks 0 days to 13 weeks 6 days
Second trimester: 14 weeks 0 days to 27 weeks 6 days
Third trimester: 28 weeks 0 days onwards

What the milestones mean

The timeline marks dates that are widely used as reference points: the estimated conception date around week 2, the missed period around week 4, the halfway mark at week 20, and the start of term at week 37.

These are calendar positions derived from your dates, not predictions about your individual pregnancy or a schedule of appointments. Your care team will set the actual schedule.

A worked example

Take a last period starting 26 June 2026, an average 28-day cycle, and today being 19 September 2026.

Due date: 26 June 2026 plus 280 days gives 2 April 2027.
Gestational age: 19 September 2026 is 85 days after the last period.
85 divided by 7 is 12 weeks with 1 day left over, so 12 weeks and 1 day.
Week 12 falls below week 14, so this is the first trimester, with 195 days to go.

The same dates would follow from entering 2 April 2027 as a known due date instead.

What this calculator can't tell you

Every date here follows from the two figures you enter. If the last period date is uncertain, the whole timeline shifts with it.
Cycle-length adjustment assumes ovulation happens a fairly consistent 14 days before your next period, which does not hold for everyone.
Milestone weeks are standard conventions, not personal medical advice or an appointment schedule.
An early ultrasound dates a pregnancy more accurately than any calculation from a last period.

What to do next

Note your due date and current week for your first appointment.
Use the pregnancy week calculator any time you want a quick check on how far along you are.
If your pregnancy came from IVF, the IVF due date calculator is more precise.

Common questions

What is the difference between this and the due date calculator?

The due date calculator answers a single question and offers three dating methods, including conception and IVF. This one takes your dates and lays out the whole picture at once: due date, current week and day, trimester, progress through the 40 weeks, and the milestone dates. Use whichever matches what you want to know.

Which trimester am I in?

Enter your dates above and the calculator will tell you. As a rule of thumb, weeks 0 to 13 are the first trimester, weeks 14 to 27 the second, and week 28 onwards the third.

How many weeks is a full-term pregnancy?

Birth from 37 weeks is considered term, and 39 to 41 weeks is described as full term. The 40-week due date sits inside that range rather than at its end, which is why arriving a week either side of it is entirely ordinary.

Can I use this if I know my due date but not my last period?

Yes. Switch the calculator to the due date option. It works backwards to the equivalent last period and produces the same timeline from there.

Does the calculator work for twins?

The dating works the same way, since gestational age is measured identically. Twin pregnancies are often delivered earlier than 40 weeks, so the due date is less useful as a guide to timing. Your care team will advise on what to expect.