Pregnancy Week Calculator
Pregnancy is counted in completed weeks from the first day of your last menstrual period, which means you are already counted as two weeks pregnant at conception.
Example: For a last period starting 1 January 2026 with a 28-day cycle, the estimated due date is 8 October 2026. On 1 March 2026 that pregnancy would be 8 weeks 3 days along, in trimester 1.
Enter a date to see your estimate.
Estimates only. Around 4% of babies are born on their estimated due date. Ultrasound dating supersedes last-period dating.
Find an OB-GYN near youHow this is calculated
Gestational age is the number of days since the dating anchor, expressed as completed weeks plus days. The second trimester begins at 14 weeks 0 days and the third at 28 weeks 0 days. Beyond 42 weeks a pregnancy is considered post-term and needs review by an OB-GYN.
This tool is for information only. It does not diagnose, treat, or replace advice from a licensed clinician. Reviewed against ACOG guidance.
Frequently Asked Questions
How many weeks pregnant am I?
Count the days from the first day of your last menstrual period to today, then express that as completed weeks plus leftover days. The calculator does the counting and applies the cycle-length adjustment for you. If you already have ultrasound dating, enter that instead â it is the more accurate anchor.
Why does pregnancy start counting before conception?
Because dating is anchored to your last menstrual period rather than to conception, and in a textbook 28-day cycle conception follows 14 days later. That is why you are counted as two weeks pregnant on the day of conception. It is a dating convention, not an error in the calculator.
What week does the second trimester start?
At 14 weeks 0 days, and the third trimester starts at 28 weeks 0 days. Those are the boundaries this calculator uses to tell you which trimester you are in. Individual clinics sometimes describe the split slightly differently, so go with what your chart says.
Why is pregnancy measured in weeks instead of months?
Because prenatal visits, screening tests and growth checks are all scheduled to specific weeks, while a calendar month is not a fixed number of days. Gestational age is written as completed weeks first, then the leftover days. Months are fine in conversation but every clinical milestone runs on weeks.
How much longer until my due date?
The calculator shows the days remaining between today and your estimated due date. Because the due date is an estimate, treat that countdown as a guide rather than a deadline. Labour starting before or after it is normal.
When should I start prenatal checkups?
As soon as you know you are pregnant. Book a first visit with an OB-GYN or your local health centre rather than waiting for a particular week. Early dating is more accurate than late dating, so an early appointment also sharpens every figure on this page.