How far along am I?
Pregnancy is counted from the first day of your last menstrual period, about two weeks before conception, and lasts 280 days, or 40 weeks, to the due date. The calculator finds your due date from the date you know, then shows how many weeks pregnant you are today, your trimester and the key dates ahead.
Example: if your last period started 10 weeks ago and your cycle is 28 days, you are 10 weeks pregnant, in the first trimester, and your due date is 30 weeks from today.
Ways to date a pregnancy
- Last period: first day of the last period + 280 days, adjusted for cycles longer or shorter than 28 days.
- Ultrasound: an early scan measures the baby and is the most accurate way; the due date follows from the length of pregnancy it shows.
- Conception date: conception + 266 days.
- IVF transfer: transfer date + 266 days − the age of the embryo (3, 5 or 6 days).
The three trimesters
- First trimester: weeks 1 to 13. The organs form; nausea and tiredness are common.
- Second trimester: weeks 14 to 27. Often the most comfortable; you start to feel the baby move.
- Third trimester: week 28 to birth. The baby gains weight and gets ready to be born.
Frequently asked questions
How accurate is a due date?
Only about 4% of babies arrive on the due date itself. Most are born between 37 and 42 weeks, and a birth within two weeks either side is normal.
Why is pregnancy counted from the last period?
The day of conception is rarely known, but most people know when their last period began. Counting from it adds about two weeks before conception.
My ultrasound date differs from my period date. Which is right?
Doctors usually go by an early ultrasound when it differs from the period date by more than about a week in the first trimester.