Pregnancy Week Calculator: How Many Weeks Pregnant Am I?

How is pregnancy week calculated?

Pregnancy week is found by dividing the number of days since the first day of your last period by seven. For example, if 73 days have passed since the first day of your last period, you are 10 weeks and 3 days pregnant (73 ÷ 7 = 10, remainder 3). The tool does this calculation for you — you only need to select the first day of your last period, and enter your average cycle length if it differs from 28 days.

The result is shown in weeks and days. When your doctor says "you're 12 weeks and 2 days pregnant," this is the pregnancy age expressed this way.

Why is pregnancy week counted from the last period date?

The exact day of fertilization is usually unknown, but the first day of the last period is generally remembered. That's why, in medicine, pregnancy age is counted from the first day of the last period. In a 28-day cycle, ovulation occurs around day 14 — meaning that by the time fertilization actually happens, the pregnancy is already considered about two weeks along.

As a result, the baby's actual age since conception is about two weeks less than the stated pregnancy week. This isn't an error — it's a shared convention so everyone uses the same measurement.

Why does knowing the week matter?

Prenatal visits and screening tests are scheduled according to pregnancy week: confirming the pregnancy and the first ultrasound in the first trimester, detailed ultrasound and screening tests in the second trimester, and monitoring the baby's growth and birth readiness in the third trimester. Knowing the correct week helps your doctor plan which test to do and when. Since the exact schedule varies by person and by how the pregnancy progresses, treat the information here as a general framework and confirm your check-up dates with your doctor.

Trimesters and week ranges

Pregnancy is divided into three periods (trimesters):

• 1st trimester: weeks 1–13 (up to 13 weeks 6 days)
• 2nd trimester: weeks 14–27
• 3rd trimester: week 28 until birth

Limitations of the result

The calculated week is an estimate based on your last period date. In women with irregular cycles, those who don't remember their last period exactly, or those who ovulate late, the actual pregnancy age may differ. The exact pregnancy week is determined by ultrasound measurements, especially in the first trimester. This tool is not a medical diagnosis — see an obstetrician for your pregnancy care.