A "how many weeks pregnant" calculator estimates the current week of pregnancy based on key dates such as:
- The first day of your last menstrual period (LMP), adding 280 days (40 weeks) from that date to estimate the due date and pregnancy week. This is the most common method. Pregnancy weeks begin counting from the first day of LMP, starting with Week 0, not the day of conception.
- The date of conception, if known, by adding 266 days (38 weeks) from that date.
- Ultrasound measurements (often used to adjust due date estimates as pregnancy progresses).
- IVF embryo transfer date, if applicable.
Typically, you enter the first day of your last period or conception date into the calculator, and it will tell you how many weeks pregnant you are and your estimated due date. The count includes the current week of gestation starting from the LMP. The expected pregnancy length is usually about 37 to 42 weeks. Such calculators also explain that due dates are estimates and that only a small percentage of babies are born exactly on the predicted date. These calculators help expectant mothers understand where they are in their pregnancy timeline, track milestones, and plan prenatal care accordingly. Examples include tools found on websites like Pampers, Babylist, NHS, and Healthify, which provide easy input fields for date of last period or conception and show weeks pregnant as output.
