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Pregnancy Weeks Calculator

Calculate weeks pregnant

The Pregnancy Weeks Calculator helps you track exactly how far along you are. It converts your dates into standard medical "pregnancy weeks" and typically aligns with what your doctor will tell you at your first prenatal visit.

Inputs Explained

First Day of Last Period (LMP): The standard medical starting point for pregnancy dating. Even though you likely conceived ~2 weeks later, dating starts here.

Result

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Formula

Weeks = Days since LMP ÷ 7
Full term = 40 weeks

Pregnancy Milestones

First trimester	Weeks 1-12
Second trimester	Weeks 13-26
Third trimester	Weeks 27-40
Full term	39-40 weeks

Complete Guide to Pregnancy Weeks

Pregnancy is universally measured in weeks, starting from the first day of your last menstrual period (LMP). This period of 40 weeks (280 days) is called your gestational age.

How to Calculate (Example)

Let's find out how many weeks pregnant someone is:

LMP Date: January 1st.
Current Date: February 14th (Valentine's Day).

Calculation:

Count the days between dates: 31 (Jan) + 14 (Feb) = 45 days.
Divide by 7: 45 ÷ 7 = 6 weeks and 3 days.

Result: Use the calculator to avoid the calendar math!

Why Do We Count from LMP?

It seems confusing to count the first two weeks before you were even pregnant! Doctors use this method because:

Certainty: The exact day of conception (ovulation) is often unknown, but most women track their periods.
Standardization: It allows all medical providers to speak the same language regarding fetal development milestones.

Use Cases

Medical Appointments: Knowing your week helps you schedule time-sensitive tests (like the 12-week scan or 20-week anatomy scan).
Tracking Development: Follow along with apps that tell you "Baby is the size of a blueberry."
Travel Planning: Most airlines restrict travel after 36 weeks.

Pregnancy Milestones by Week

🌱 Week 4: The missed period. Positive home pregnancy test.
❤️ Week 6-7: Heartbeat may be detectable by ultrasound.
👶 Week 12: End of the first trimester. Risk of miscarriage drops significantly.
🦶 Week 20: Anatomy scan. Gender reveal. You may feel movement.
🎉 Week 37: Early term. Baby is considered ready for birth from here on.

Medical Disclaimer: This calculator provides an estimate based on regular 28-day cycles. Early ultrasounds are the most accurate method for dating a pregnancy. Always consult your OB/GYN or midwife for official dating.

Frequently Asked Questions

Medical professionals count pregnancy from the first day of your last menstrual period (LMP). This means you are considered 4 weeks pregnant about 2 weeks after you actually conceive.

Because the exact date of conception is hard to pinpoint, doctors use the first day of the last period as a reliable starting point. This standard dating method is 40 weeks long.

Full term is considered 39 to 40 weeks. Babies born between 37 and 42 weeks are generally considered 'term' babies.

The second trimester begins at week 13 and runs through week 26. This is often when morning sickness subsides and you start to feel the baby move.

Yes. If your early ultrasound measurements differ significantly from your LMP dating (usually by more than 7 days), your provider may adjust your due date and 'gestational age'.

References

Gestational age standards are based on clinical guidelines:

American College of Obstetricians and Gynecologists (ACOG). "Method for Estimating Due Date".
Centers for Disease Control and Prevention (CDC). "Pregnancy Stages and Milestones".
Cleveland Clinic. "Fetal Development: Stages of Growth".