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How Many Weeks Pregnant Am I?
Enter your last-period date or your due date, and we’ll show exactly how many weeks and days pregnant you are today — with trimester, baby’s size this week, and the countdown to full term and birth.
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Gestational age vs fetal age
Gestational age is measured from the first day of your last menstrual period. Fetal age is measured from conception, which is roughly 2 weeks later. Doctors use gestational age as the standard — so if you are 20 weeks pregnant, your baby is approximately 18 weeks old.
How weeks are counted
Pregnancy is divided into 40 weeks from LMP, broken into three trimesters:
- First trimester — weeks 1 through 12.
- Second trimester — weeks 13 through 27.
- Third trimester — weeks 28 through 40.
Why it isn’t the same as months
Forty weeks divided by 4 gives 10 months, not 9. The 9-month folk count reflects calendar months (28–31 days each), while pregnancy weeks are strictly 7 days. Most apps and hospitals count in weeks to avoid confusion.
Frequently asked questions
Why does my app say 12 weeks when my baby is 10 weeks old?
Apps use gestational age, which is 2 weeks ahead of conception age. A “12-week” pregnancy contains a 10-week-old baby.
How do I know my LMP if my cycles are irregular?
An early dating ultrasound (8–13 weeks) measures the baby directly and is more reliable than LMP if your cycles are unpredictable.
Is the week count different in twin pregnancies?
No — weeks are counted the same way, but many multiples are delivered preterm (around 36–37 weeks).
What happens after 40 weeks?
Around 58% of first-time mothers deliver after their due date. Most providers offer induction by 42 weeks.
Is this calculator stored anywhere?
No. All calculations happen in your browser locally.
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Calculations based on standard obstetric formulas and WHO guidance. For medical decisions, always consult your doctor or midwife.