About the Pregnancy Week Calculator
This pregnancy week calculator answers “how many weeks pregnant am I?” in seconds. Enter the first day of your last period, your due date or a known conception date and it shows your exact gestational age in weeks and days, which trimester you are in, the percentage of pregnancy completed and a week-by-week timeline of what is coming up.
Gestational age is the number your midwife, doctor and ultrasound reports use. It starts from the first day of your last menstrual period, so at conception you are already about two weeks pregnant. Trimesters are counted the way ACOG does: the first runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks until birth.
Use it to plan appointments, prepare for scans and screening tests, or simply to follow along. You can also change the “as of” date to see exactly how far along you will be at a future event, a trip or the start of maternity leave.
How to use the pregnancy week calculator
- 1Choose whether you know your last period, your due date or conception date.
- 2Enter that date (and your cycle length if using your last period).
- 3Keep the as-of date on today or set a future date.
- 4Read your weeks and days, trimester and percent complete.
- 5Check the timeline table for upcoming scans and screening windows.
Formula and method
The calculator first finds gestational day 0. With your last period it uses the LMP, shifted by your cycle length minus 28 days. With a due date it counts back 280 days, and with a conception date it counts back 14 days. The number of days from day 0 to the as-of date is your gestational age, split into whole weeks and leftover days.
Trimesters follow ACOG definitions (first: up to 13 weeks 6 days; second: 14w0d–27w6d; third: 28w0d onward). Percent complete is gestational days ÷ 280. The fruit-size comparison is a rough, commonly used visual guide for average fetal size, not a measurement.
- LMP
- First day of the last menstrual period
- 280
- Days in a 40-week pregnancy
Worked examples
Last period June 1, 2026, checked on September 29
From June 1 to September 29, 2026 is 120 days, which is 17 weeks 1 day. That is the second trimester and 120 ÷ 280 = 42.9% of the way through. The due date is 280 days after June 1: March 8, 2027, 160 days away.
Know only the due date
A December 25, 2026 due date puts day 0 on March 20, 2026. September 29 is 193 days later — 27 weeks 4 days — still the second trimester, with 87 days to go.
Conception date known
Conception on August 20 means gestational day 0 is August 6. By September 29 you are 54 days, or 7 weeks 5 days, pregnant, and the due date is 266 days after conception: May 13, 2027.
Frequently asked questions
How are pregnancy weeks counted?+
Weeks are counted from the first day of your last menstrual period, not from conception. So at the moment of conception you are already considered about 2 weeks pregnant, and a full-term pregnancy is 40 weeks.
What weeks are each trimester?+
The first trimester is from week 0 to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until delivery, which is usually around 40 weeks.
How many months pregnant is 20 weeks?+
Twenty weeks is roughly 4½ months, the halfway point of a 40-week pregnancy. Months are an imprecise way to count because they vary in length, which is why providers use weeks and days.
What does “full term” mean?+
ACOG defines early term as 37w0d–38w6d, full term as 39w0d–40w6d, late term as 41w0d–41w6d and post-term as 42 weeks or more. Babies born before 37 weeks are preterm.
Why does my scan say a different number of weeks?+
An ultrasound estimates age from the baby’s size. If an early scan differs from your period-based dates by more than about a week, your provider may re-date the pregnancy and the scan dates take priority.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.