Find out how many weeks pregnant you are and which trimester you're in, based on your last period.
Calculate your pregnancy progress instantly.
Pregnancy is measured from the first day of your last menstrual period (LMP), not from the date of conception — which means you're already considered about two weeks pregnant before conception typically occurs.
This method is used because conception dates are rarely known precisely, while the start of a period is usually easy to track. A full-term pregnancy is calculated as 40 weeks from this starting point, divided into three trimesters.
| Trimester | Weeks | What's Happening |
|---|---|---|
| First | 1 – 12 | Major organ development |
| Second | 13 – 26 | Rapid growth, movement felt |
| Third | 27 – 40 | Final growth, preparing for birth |
Only about 5% of babies are born exactly on their calculated due date. Most arrive within a window of one to two weeks before or after, since the 40-week estimate is an average, not a guarantee tied to your specific pregnancy.
If you know your conception date more precisely (e.g. through IVF), your healthcare provider can adjust your estimated dates accordingly.
Because conception dates are rarely known exactly, doctors use the last menstrual period as a consistent, trackable starting point for all pregnancies.
No. This tool gives a general estimate only. Always follow up with regular prenatal appointments and ultrasounds for accurate, personalized dating.
Irregular cycles can make LMP-based calculations less accurate. An early ultrasound is typically the most reliable way to confirm gestational age in this case.