How to use the pregnancy week calculator
- Choose what you know: the first day of your last period, your due date (for example from an ultrasound), or your conception date.
- Enter the date.
- See how many weeks and days pregnant you are, your trimester, your due date and the dates of upcoming milestones.
How pregnancy weeks are counted
Doctors count pregnancy in gestational weeks starting from the first day of your last menstrual period (LMP) — about two weeks before conception. A full pregnancy is 40 weeks (280 days) from the LMP, or 38 weeks from conception.
Example: if your last period started 70 days ago, you are 70 ÷ 7 = 10 weeks, 0 days pregnant — in your first trimester, in week 11 of 40.
"10 weeks pregnant" vs. "week 11"
When you are 10 weeks and 3 days pregnant, you have completed 10 weeks and are in your 11th week. Apps and books sometimes use either form, which is why week counts can seem off by one.
Baby's size by week (approximate)
| Week | Average length | Average weight | About the size of |
|---|---|---|---|
| 8 | 1.6 cm (0.6 in) | 1 g | a raspberry |
| 12 | 5.4 cm (2.1 in) | 14 g (0.5 oz) | a lime |
| 16 | 11.6 cm (4.6 in) | 100 g (3.5 oz) | an avocado |
| 20 | 25.6 cm (10.1 in)* | 300 g (10.6 oz) | a banana |
| 24 | 30 cm (11.8 in) | 600 g (1.3 lb) | an ear of corn |
| 28 | 37.6 cm (14.8 in) | 1 kg (2.2 lb) | an eggplant |
| 32 | 42.4 cm (16.7 in) | 1.7 kg (3.8 lb) | a squash |
| 36 | 47.4 cm (18.7 in) | 2.6 kg (5.8 lb) | a honeydew melon |
| 40 | 51 cm (20.1 in) | 3.4 kg (7.6 lb) | a small watermelon |
*Before 20 weeks length is usually measured crown to rump; from 20 weeks, crown to heel. Every baby grows at its own pace, so your measurements at scans may differ.
Trimesters
- First trimester: weeks 1 to 13 — organs form; nausea and tiredness are common.
- Second trimester: weeks 14 to 27 — many people feel better and notice the first movements.
- Third trimester: weeks 28 to 40+ — the baby gains most of its weight.
Need just the due date? Use the due date calculator.
Frequently asked questions
How do I calculate how many weeks pregnant I am?
Count the days from the first day of your last menstrual period to today and divide by 7. The whole number is the completed weeks and the remainder is extra days.
Why does pregnancy start before conception?
By convention, pregnancy is dated from the first day of your last period because that date is usually known, while the exact day of conception usually is not. Conception typically happens about two weeks later.
How many weeks is each trimester?
The first trimester runs through week 13, the second from week 14 to week 27, and the third from week 28 until birth, usually around week 40.
What month of pregnancy am I in?
Divide the weeks by about 4.35. At 20 weeks you are in your fifth month. A 40-week pregnancy is a little over nine calendar months.
Can my due date change?
Yes. If an early ultrasound measurement differs from the date based on your last period, your provider may update your due date, and the week count changes with it.
This calculator provides general information only and is not a substitute for professional medical advice, diagnosis or treatment. Always consult a doctor or other qualified health provider about your individual situation.
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