Pregnancy Week Calculator

Pregnancy Calculator

Find out how many weeks pregnant you are, when your baby is due, and what's happening with your baby right now - free, private, instant.

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Medical review: Dr. Sarah Mitchell, MD · Board-certified OB-GYN · Updated March 2026

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How It Works

How Pregnancy Weeks Are Counted

Pregnancy weeks are counted from the first day of the last menstrual period, not from the day of conception. That is why a 40-week pregnancy is usually about 38 weeks from fertilization.

Doctors use weeks instead of months because weeks are precise. Screening windows, ultrasound dating, anatomy scans, glucose testing, and term definitions all depend on week-level timing.

Pregnancy Month	Pregnancy Weeks	Stage
Month 1	Weeks 1-4	First trimester
Month 2	Weeks 5-8	First trimester
Month 3	Weeks 9-13	First trimester
Month 4	Weeks 14-17	Second trimester
Month 5	Weeks 18-22	Second trimester
Month 6	Weeks 23-26	Second trimester
Month 7	Weeks 27-30	Third trimester
Month 8	Weeks 31-35	Third trimester
Month 9	Weeks 36-40	Third trimester

The 4 Ways to Calculate Your Due Date

The last-period method uses Naegele's Rule: add 280 days, or 40 weeks, to the first day of the last menstrual period. For longer or shorter cycles, the estimate shifts by the cycle-length difference from 28 days.

"Due date = LMP + 280 days + (cycle length - 28 days)"

The conception-date method adds 266 days. Ultrasound dating uses the scan date and measured gestational age; first-trimester ultrasound is often accurate within about 5 to 7 days. IVF dating uses known embryo age: Day 3 transfer plus 263 days, Day 5 transfer plus 261 days.

How Accurate Is a Due Date?

A due date is the center of a birth window, not an appointment. Only a small share of babies arrive on the exact due date, while many arrive within about 2 weeks before or after.

Term language helps set expectations. Early term, full term, late term, and post-term describe different levels of readiness and monitoring needs.

Term Label	Weeks	Meaning
Early term	37w0d-38w6d	Often healthy, but still earlier than full term
Full term	39w0d-40w6d	The preferred term window for many uncomplicated pregnancies
Late term	41w0d-41w6d	Closer monitoring or planning may be discussed
Post-term	42w0d and later	Requires clinician-led monitoring and next-step planning

Frequently Asked Questions

Count from the first day of your last menstrual period to today, then divide by 7. The whole number is your pregnancy week and the remainder is the day within that week. This calculator does that automatically and can also use conception date, ultrasound dating, or IVF transfer date.

A first-trimester ultrasound is usually the most accurate mainstream dating method, often within about 5 to 7 days. IVF transfer dating can be even more precise because embryo age is known. LMP dating works best when cycles are regular.

Yes, but LMP-based results are less precise when cycles are irregular because ovulation may not happen on the expected day. Use the ultrasound tab when you have a dating scan, or review the estimate with your prenatal clinician.

Gestational age counts from the first day of the last period and is the standard used by clinicians. Fetal age counts from conception and is usually about 2 weeks less than gestational age.

IVF due date calculation starts with the embryo transfer date and embryo age. A day 5 transfer adds 261 days, a day 3 transfer adds 263 days, and a day 2 transfer adds 264 days.

Many home pregnancy tests can turn positive around the time a period is missed, roughly 2 weeks after ovulation. Testing too early can give a false negative, so repeat testing or clinical confirmation may be needed.

Full term usually means 39 weeks through 40 weeks and 6 days. Early term is 37 weeks through 38 weeks and 6 days, late term is 41 weeks through 41 weeks and 6 days, and post-term is 42 weeks or beyond.

The last period date is often easier to identify than the exact conception date. Counting from LMP gives clinicians a shared clock for scans, screening windows, trimester timing, and delivery planning.

Yes. If an early ultrasound differs meaningfully from the LMP estimate, clinicians may update the due date. First-trimester scans are especially useful when periods are irregular or the LMP date is uncertain.

The first trimester runs through week 13, the second trimester runs from weeks 14 to 26, and the third trimester begins at week 27 and continues until birth.

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Sources

Medical references and methodology

This pregnancy calculator uses standard pregnancy dating rules and public clinical references. The review label matches the rest of the site: Board-certified OB-GYN with 15+ years of clinical experience. Last reviewed: March 17, 2026. This page is educational and is not medical advice.