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Pregnancy Calculator

Enter your last period or due date to see how many weeks pregnant you are, your trimester and dates for the important milestones.

Formulas last reviewed: September 2026

Disclaimer: Results are general estimates for information and education only. They are not medical advice, a diagnosis or a treatment plan. Please talk to a doctor or qualified health professional before making health, diet, exercise or pregnancy decisions.

How is pregnancy counted?

Doctors count a pregnancy from the first day of your last menstrual period (LMP), not from conception. That is why you are already “2 weeks pregnant” when conception happens, and why a full pregnancy is 40 weeks.

This calculator turns your last period, conception date, IVF transfer, ultrasound or due date into how many weeks you are, your trimester and the dates of the main milestones.

How it is calculated

Due date = first day of last period + 280 days
Gestational age = today − first day of last period

Pregnancy is split into three trimesters: weeks 1–13, 14–27 and 28 until birth. A pregnancy is full term from 37 weeks.

Worked example

Last period started on 1 January 2026

Due date
Thu, 8 Oct 2026
On 12 March 2026
10 weeks 0 days (trimester 1)
End of first trimester
Week 13 + 6 days
Full term begins
Week 37

Key milestones

MilestoneWhen
Pregnancy test reliableWeek 4 (missed period)
Heartbeat on ultrasoundWeeks 6–7
Anatomy scanWeeks 18–22
Baby considered viableWeek 24
Full termWeek 37
Due dateWeek 40

This is an estimate for information. Your doctor's scans and check-ups give the most reliable dating and care.

Frequently asked questions

Why is pregnancy counted from the last period?

The date of conception is usually not known exactly, while the first day of the last period usually is. Counting from it gives a consistent starting point.

How accurate is the due date?

Only about 1 in 20 babies is born on the due date. Most arrive between 37 and 42 weeks. Early ultrasound dating is the most accurate.

What if my cycle is not 28 days?

Enter your average cycle length. A longer or shorter cycle moves ovulation and the due date by the difference from 28 days.

When should I see a doctor?

As soon as you have a positive pregnancy test. Early antenatal care, folic acid and scans help protect you and your baby.