How is the pregnancy due date calculated?

The most common method uses Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a regular 28-day cycle with ovulation on day 14. From a known conception date, add 266 days (38 weeks). For IVF transfers, the calculation accounts for the embryo age at transfer.

How accurate is the due date?

Only about 5% of babies are born on their estimated due date. Most are born within 1-2 weeks before or after. First-trimester ultrasound dating (7-13 weeks) is the most accurate method, within ±5-7 days. LMP-based calculations can be off by 2 weeks or more, especially with irregular cycles.

What is Naegele's rule?

Naegele's rule estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). An alternative form: add 7 days to the LMP, subtract 3 months, then add 1 year. Named after German obstetrician Franz Naegele (1778-1851). It assumes a 28-day cycle.

How is the IVF due date different?

IVF due dates account for the embryo's age at transfer. For a day-3 transfer, the embryo is already 3 days old at transfer, so the effective LMP is 17 days before transfer (14 + 3). For a day-5 blastocyst transfer, the LMP is 19 days before transfer (14 + 5). This makes IVF due dates more precise since the conception date is known exactly.

What are the three trimesters of pregnancy?

The first trimester (weeks 1-12) involves major organ formation and highest miscarriage risk. The second trimester (weeks 13-26) brings the anatomy scan, first kicks, and viability at 24 weeks. The third trimester (weeks 27-40) features rapid growth, lung maturity, and preparation for delivery. Full term begins at 37 weeks.

When is a baby considered full term?

Early term is 37-38 weeks, full term is 39-40 weeks, late term is 41 weeks, and post-term is 42+ weeks. The American College of Obstetricians and Gynecologists recommends against elective delivery before 39 weeks. Most doctors discuss induction if pregnancy extends past 41 weeks.

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Due Date Calculator

Pregnancy due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). From conception, add 266 days (38 weeks). For IVF, count from transfer date minus embryo age. Pregnancy has three trimesters: weeks 1-12, 13-26, and 27-40. Viability is at 24 weeks, full term at 37 weeks. Only about 5% of babies arrive on the due date.

Calculate your pregnancy due date from last menstrual period (LMP), conception date, or IVF transfer date using Naegele's rule. Track gestational age in weeks and days, trimester progress, week-by-week fetal development with size comparisons, key milestone dates including viability and full term, and countdown to delivery.

Calculation Method

Last Menstrual Period

First day of your last menstrual period

How to Use

Enter your value in the input field
Click the Calculate/Convert button
Copy the result to your clipboard

Frequently Asked Questions

How is the pregnancy due date calculated?: The most common method uses Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a regular 28-day cycle with ovulation on day 14. From a known conception date, add 266 days (38 weeks). For IVF transfers, the calculation accounts for the embryo age at transfer.
How accurate is the due date?: Only about 5% of babies are born on their estimated due date. Most are born within 1-2 weeks before or after. First-trimester ultrasound dating (7-13 weeks) is the most accurate method, within ±5-7 days. LMP-based calculations can be off by 2 weeks or more, especially with irregular cycles.
What is Naegele's rule?: Naegele's rule estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). An alternative form: add 7 days to the LMP, subtract 3 months, then add 1 year. Named after German obstetrician Franz Naegele (1778-1851). It assumes a 28-day cycle.
How is the IVF due date different?: IVF due dates account for the embryo's age at transfer. For a day-3 transfer, the embryo is already 3 days old at transfer, so the effective LMP is 17 days before transfer (14 + 3). For a day-5 blastocyst transfer, the LMP is 19 days before transfer (14 + 5). This makes IVF due dates more precise since the conception date is known exactly.
What are the three trimesters of pregnancy?: The first trimester (weeks 1-12) involves major organ formation and highest miscarriage risk. The second trimester (weeks 13-26) brings the anatomy scan, first kicks, and viability at 24 weeks. The third trimester (weeks 27-40) features rapid growth, lung maturity, and preparation for delivery. Full term begins at 37 weeks.
When is a baby considered full term?: Early term is 37-38 weeks, full term is 39-40 weeks, late term is 41 weeks, and post-term is 42+ weeks. The American College of Obstetricians and Gynecologists recommends against elective delivery before 39 weeks. Most doctors discuss induction if pregnancy extends past 41 weeks.

Pregnancy Trimester Breakdown

Trimester	Weeks	Key Developments
1st	Weeks 1–12	Implantation, heartbeat, major organs form
2nd	Weeks 13–26	Movement felt, anatomy scan, viability at 24 weeks
3rd	Weeks 27–40	Rapid growth, lung maturity, preparation for birth

Due Date Calculation Methods

Method	Formula	Best For
LMP (Naegele's Rule)	LMP + 280 days	Regular 28-day cycles
Conception Date	Conception + 266 days	Known ovulation date
IVF Day-3 Transfer	Transfer - 17 + 280 days	Day-3 embryo transfers
IVF Day-5 Transfer	Transfer - 19 + 280 days	Blastocyst transfers
Ultrasound Dating	Measured by doctor	Most accurate (1st trimester)

Key Pregnancy Milestones

Week	Milestone	Significance
6	Heartbeat	First detectable heartbeat on ultrasound
12	End of 1st Trimester	Miscarriage risk drops significantly
20	Anatomy Scan	Detailed ultrasound; halfway point
24	Viability	Baby can survive outside womb with medical care
37	Full Term	Baby is fully developed
40	Due Date	Estimated delivery (only 5% deliver on this date)
42	Post-Term	Induction typically discussed