Pregnancy Week by Week
A clear, week-by-week guide to the whole pregnancy — how big your baby is, what’s developing, the symptoms to expect, and practical tips, from week 1 all the way to 42. Pick a week below.
First trimester
Weeks 1–13
Second trimester
Weeks 14–27
How pregnancy weeks work
Pregnancy is counted in weeks from the first day of your last menstrual period (LMP), not from conception — which is why the count starts about two weeks before you actually conceive. A full pregnancy runs around 40 weeks, divided into three trimesters, each with its own developmental milestones and changes for you.
Use this guide alongside our Pregnancy Due Date Calculator to find which week you’re in today, and the Pregnancy Test Calculator if you’re still at the very beginning.
Frequently asked questions
- How many weeks is a full pregnancy?
- A full-term pregnancy is about 40 weeks, counted from the first day of your last menstrual period. 'Term' covers weeks 37 to 42: early term (37–38), full term (39–40), late term (41), and post term (42+). Only about 1 in 20 babies arrive exactly on the 40-week due date.
- Why does pregnancy start counting before conception?
- Pregnancy is dated from the first day of your last period (LMP) because that date is far easier to pin down than the exact moment of conception. It means weeks 1 and 2 happen before you're actually pregnant — conception occurs around the end of week 2, and implantation in week 4.
- What are the three trimesters?
- The first trimester is weeks 1–13 (organs form, symptoms peak), the second is weeks 14–27 (the 'honeymoon' stretch, anatomy scan, first movements), and the third is weeks 28 to birth (rapid growth and preparation for delivery).
- How accurate is the baby-size-by-week comparison?
- The fruit-and-vegetable size comparisons are a fun, rough guide based on average length and weight at each week. Real babies vary widely and healthily. Your provider measures your baby's individual growth at ultrasound scans, which is what actually matters.