Gestational age calculator tools estimate how many weeks pregnant a person is by using the first day of the last menstrual period. This method defines gestational age as the number of weeks and days counted from that date.
By entering the last menstrual period, a gestational age calculator shows how far the pregnancy has progressed and how it fits into trimesters and months. This calculation is commonly used to track pregnancy length and support due date estimates.
Ultrasound exams can also determine gestational age and are considered the most accurate option when the last menstrual period is unknown. Gestational age calculations help estimate the due date, understand normal pregnancy duration, and guide prenatal care and delivery planning.
Online calculator
Use our calculator below to determine how many weeks pregnant you may be:
This calculator helps you find out the number of weeks and days that have passed since the start of your pregnancy simply by entering the date of your last period.
What if I don't know the date of my last period?
If you’re unsure about the date of your last period, the most accurate way to determine how many weeks pregnant you are is to see your OB/GYN or midwife, who can estimate your gestational age using an ultrasound.
Also recommended: Period Tracker: Determine When Your Period is Due tuasaude.com/en/period-trackerHow many weeks pregnant am I?
Pregnancy timing is tracked in weeks + days (for example, 39 weeks and 2 days). Months are only an approximation, and the exact week range can vary slightly from one source to another.
Pregnancy is often described as lasting about 40 weeks from the first day of the last menstrual period, but delivery timing is classified more precisely according to the American College of Obstetricians and Gynecologists (ACOG):
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Early term: 37 0/7–38 6/7 weeks;
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Full term: 39 0/7–40 6/7 weeks;
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Late term: 41 0/7–41 6/7 weeks;
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Postterm: 42 0/7 weeks and beyond.
If labor hasn’t begun by about 41 weeks, ACOG notes that planning delivery (often by induction) may be recommended or considered to reduce risks that increase with late-term and postterm pregnancy, depending on factors like dating certainty and maternal-fetal health.
Also recommended: Pregnancy Week by Week: Maternal Changes & Fetal Development tuasaude.com/en/pregnancy-week-by-weekHow to calculate your due date
To calculate your expected date of delivery (due date), which is about 40 weeks after the first day of your last menstrual period (LMP), add 7 days to your LMP, then count back 3 months and add 1 year.
For example, if the LMP was January 15, 2026, add 7 days, (January 22, 2026), then subtract 3 months (October 22, 2025) and add 1 year (October 22, 2026). In this case, the estimated due date is October 22, 2026.
This calculation is not 100% accurate, as the baby can be born between 37 and 42 weeks of pregnancy, however, it allows the parents and health care team to prepare for delivery.