How is a due date calculated?

The standard rule (Naegele's rule) adds 280 days, or 40 weeks, to the first day of your last menstrual period, assuming a 28-day cycle. If your cycle is longer or shorter, the date shifts by the difference. From a known conception date add 266 days; after IVF add 263 days to a day-3 embryo transfer or 261 days to a day-5 blastocyst transfer.

Why am I already 2 weeks pregnant at conception?

Pregnancy is dated from the first day of the last period, not from conception. Ovulation and fertilization usually happen about two weeks later, so at the moment of conception the gestational age is already about 2 weeks.

My doctor gave me a different due date. Which is right?

Use your clinician's date. ACOG recommends that an early ultrasound be used to confirm or change the due date when it differs from the period-based date by more than a set margin, and that the date not be changed later without good reason. Enter your ultrasound result in the Ultrasound tab to recalculate on the same basis.

Pregnancy Due Date Calculator

Enter your last period, conception date, IVF transfer or an ultrasound result to see your estimated due date, how far along you are, your trimester and the dates of common prenatal tests.

Pregnancy summary — honglife.co.kr

Estimated due date (40 weeks 0 days)

—

Enter a date to see your results.

0%40 weeks

--WEEKS

--DAYS

--TOTAL DAYS LEFT

Weeks pregnant	—
Trimester	—
Estimated conception	—
Early term begins (37w 0d)Most babies arrive between 37 and 42 weeks	—
Full term (39w 0d – 40w 6d)	—
42 weeks (post-term)	—

Not medical advice. This is an estimate based on an average 280-day pregnancy. Follow the due date set by your doctor or midwife, especially if it came from an early ultrasound. Contact your care team right away about bleeding, severe pain, fluid leaking or reduced baby movements, whatever the date.

How the due date is calculated

Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. Ovulation usually happens about two weeks later, so at conception you are already “2 weeks pregnant”. An average pregnancy lasts 280 days (40 weeks) from the LMP.

Last period	LMP + 280 days + (cycle − 28)
Conception	Conception + 266 days
IVF, day-3 embryo	Transfer + 263 days
IVF, day-5 blastocyst	Transfer + 261 days
Ultrasound	Scan date − measured age + 280 days
Trimesters	1st to 13w6d · 2nd 14w0d–27w6d · 3rd 28w0d+

ACOG advises that the most accurate dating comes from an IVF transfer date or a first-trimester ultrasound measurement (crown–rump length). If an early scan differs from the LMP date by more than a set margin — for example more than 5 days before 9 weeks or more than 7 days from 9 to 13 weeks — the ultrasound date is used. Everything is calculated in your browser; nothing you enter is sent anywhere.

Sources

ACOG Committee Opinion No. 700, Methods for Estimating the Due Date — SMFM summary; IVF +263/+261 days and redating thresholds: StatPearls, Estimated Date of Delivery
NICHD, What tests might I need during pregnancy? — cell-free DNA from 10 weeks, nuchal translucency 11–14 weeks, maternal serum screen 15–20 weeks, glucose screening 24–28 weeks — nichd.nih.gov
ACOG Committee Opinion No. 797, Prevention of Group B Streptococcal Early-Onset Disease in Newborns — GBS culture at 36 0/7–37 6/7 weeks — SMFM summary
ACOG Committee Opinion No. 579, Definition of Term Pregnancy — early term 37w, full term 39w0d–40w6d, late term 41w, post-term 42w — SMFM summary

Planning a pregnancy? Try the ovulation & fertile window calculator. 한국어: 출산 예정일 계산기.

Frequently asked questions

How is a due date calculated?: The standard rule (Naegele's rule) adds 280 days, or 40 weeks, to the first day of your last menstrual period, assuming a 28-day cycle. If your cycle is longer or shorter, the date shifts by the difference. From a known conception date add 266 days; after IVF add 263 days to a day-3 embryo transfer or 261 days to a day-5 blastocyst transfer.
Why am I already 2 weeks pregnant at conception?: Pregnancy is dated from the first day of the last period, not from conception. Ovulation and fertilization usually happen about two weeks later, so at the moment of conception the gestational age is already about 2 weeks.
My doctor gave me a different due date. Which is right?: Use your clinician's date. ACOG recommends that an early ultrasound be used to confirm or change the due date when it differs from the period-based date by more than a set margin, and that the date not be changed later without good reason. Enter your ultrasound result in the Ultrasound tab to recalculate on the same basis.

Your prenatal timeline

How the due date is calculated

Sources

Frequently asked questions