Pregnancy Due Date Calculator

Calculate Your Due Date

Select your calculation method below and enter the relevant date. Your estimated due date and pregnancy milestones will appear instantly.

Pregnancy Milestones by Week

This table summarizes key developmental milestones and approximate fetal size throughout pregnancy. Your current week will be highlighted when results are displayed.

How Due Dates Are Calculated

Naegele's Rule (LMP Method)

The most widely used method for estimating a due date is Naegele's rule, developed by German obstetrician Franz Karl Naegele in the early 1800s. The rule is straightforward: take the first day of the last menstrual period (LMP) and add 280 days (40 weeks). An equivalent way to think about it is to add one year, subtract three months, and add seven days.

This method assumes a 28-day menstrual cycle with ovulation occurring on day 14. Since pregnancy is dated from the LMP rather than conception, the first two weeks of the "40-week pregnancy" occur before the egg is actually fertilized.

Conception Date Method

If you know your ovulation or conception date, the calculation is more direct: add 266 days (38 weeks) from conception. This is equivalent to the LMP method because 280 - 14 = 266 days. This approach can be more accurate for women with irregular cycles.

IVF Transfer Date Method

For IVF pregnancies, the date of embryo transfer is precisely known. The calculation adjusts for the age of the embryo at transfer:

• 3-day embryo transfer: Due date = transfer date + 263 days. The embryo is already 3 days old at transfer, so 266 - 3 = 263.
• 5-day blastocyst transfer: Due date = transfer date + 261 days. The embryo is 5 days old, so 266 - 5 = 261.

Accuracy of Due Date Estimates

A due date is an estimate, not a guarantee. Studies show that only about 4-5% of babies are born on their exact due date. The majority arrive within a two-week window around the estimate. First-trimester ultrasound dating, which measures the crown-rump length of the embryo, is generally considered the most accurate method and may differ from the LMP-based date by a week or more.

The American College of Obstetricians and Gynecologists (ACOG) defines the following terms:

• Early term: 37 weeks 0 days through 38 weeks 6 days
• Full term: 39 weeks 0 days through 40 weeks 6 days
• Late term: 41 weeks 0 days through 41 weeks 6 days
• Post-term: 42 weeks 0 days and beyond

Examples

Frequently Asked Questions

How is a pregnancy due date calculated?

The most common method is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If you know your conception date, the due date is 266 days (38 weeks) from conception.

How accurate is a due date calculator?

Due date calculators provide an estimate. Only about 4-5% of babies are born on their exact due date. Most babies arrive within a two-week window around the estimated date, between 38 and 42 weeks of pregnancy. An ultrasound in the first trimester is generally considered the most accurate method for dating a pregnancy.

What is Naegele's rule?

Naegele's rule is a standard method for estimating pregnancy due dates, developed by German obstetrician Franz Naegele in the early 1800s. The rule adds 280 days (40 weeks) to the first day of the last menstrual period, or equivalently adds one year, subtracts three months, and adds seven days.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1-12, the second trimester spans weeks 13-27, and the third trimester spans weeks 28-40. Each trimester is associated with different stages of fetal development and different pregnancy symptoms.

When is a baby considered full term?

According to ACOG, early term is 37 weeks 0 days through 38 weeks 6 days, full term is 39 weeks 0 days through 40 weeks 6 days, late term is 41 weeks 0 days through 41 weeks 6 days, and post-term is 42 weeks 0 days and beyond.

How is an IVF due date calculated?

For IVF pregnancies, the due date is calculated from the embryo transfer date. For a 3-day embryo transfer, add 263 days. For a 5-day blastocyst transfer, add 261 days. This accounts for the embryo already being 3 or 5 days old at the time of transfer.

Does this calculator store my data?

No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.

Privacy & Limitations

Privacy: This calculator runs entirely in your browser. No personal data -- including dates entered or results calculated -- is transmitted or stored anywhere.

Limitations: This tool provides an estimate based on standard formulas. It does not account for individual cycle length variations, medical conditions, or other factors that can affect pregnancy dating. It is not a substitute for professional prenatal care. Always consult your healthcare provider for personalized medical guidance.

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