How is a pregnancy due date calculated?

The standard method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is called Naegele's rule. Only about 5% of babies arrive on their exact due date, most arrive within a two-week window around it.

How accurate are due date calculators?

LMP-based calculations are accurate to within about 2 weeks for most women. An early ultrasound (before 12 weeks) is more precise, accurate to within 3-5 days. Due dates become less reliable when menstrual cycles are irregular.

What if I do not know the date of my last period?

If you know your conception date (or approximate ovulation date), use the conception-based calculation which adds 266 days (38 weeks). If neither date is known, an early ultrasound is the most accurate method, it measures the embryo to estimate gestational age within 3-5 days during the first trimester.

What are the trimesters and when do they start?

The first trimester spans weeks 1-12 and involves early development, morning sickness, and the highest risk of miscarriage. The second trimester (weeks 13-26) is often called the easiest, energy returns and the baby grows rapidly. The third trimester (weeks 27-40) brings significant weight gain, preparation for delivery, and increasing discomfort.

Can my due date change during pregnancy?

Yes. Your healthcare provider may adjust the due date after an early ultrasound if the measurement differs from the LMP calculation by more than 7 days. Once set by ultrasound, the due date typically is not changed again. About 70% of babies are born within 10 days of their due date.

Pregnancy Due Date Calculator

Estimate your baby's due date based on the first day of your last menstrual period or your conception date. See your current week, trimester, and a visual timeline of your pregnancy.

By InstaCalcs Team·Reviewed April 25, 2026·Report an issue

First Day of Last Period

How to use

Choose whether to calculate from your last menstrual period (LMP) or from your conception date. Enter the appropriate date and click Calculate Due Date. The calculator will show your estimated due date, how many weeks pregnant you are, which trimester you are in, and how many days remain until your due date.

Formula, Naegele's Rule

From LMP: Due Date = LMP + 280 days (40 weeks)

From Conception: Due Date = Conception Date + 266 days (38 weeks)

Naegele's rule is the standard method used by healthcare providers to estimate a due date. It assumes a 28-day menstrual cycle with ovulation occurring on day 14. The first trimester spans weeks 1-12, the second trimester weeks 13-26, and the third trimester weeks 27-40. Only about 5% of babies are born on their exact due date, most arrive within two weeks before or after.

When this calculator helps

Knowing your estimated due date is the foundation of prenatal care planning. It helps you schedule important prenatal appointments, plan maternity or paternity leave from work, and prepare your home for the baby's arrival. Healthcare providers use the due date to track fetal development milestones and determine whether the baby is growing on schedule. This calculator gives you an estimate that matches the same formula doctors use, along with trimester information and a countdown to your due date. It works whether you know your last period date or your conception date.

Examples

Example 1: Calculating from Last Period

Last menstrual period started on March 1st. Adding 280 days gives a due date of December 6th. At 10 weeks (May 10th), you would be in the first trimester with 210 days remaining. The baby's heartbeat is typically detectable by ultrasound at this stage, and the first prenatal visit usually occurs around this time.

Example 2: Calculating from Conception Date

If conception occurred on April 15th, adding 266 days gives a due date of January 6th. This method is useful for IVF patients who know the exact transfer date, or for those who tracked ovulation precisely. Note that gestational age is counted from 2 weeks before conception (the assumed LMP date).

Example 3: Planning Maternity Leave

With a due date of October 15th, many employers allow leave to start 1-2 weeks before the due date (around October 1st). Standard FMLA provides 12 weeks of protected leave, ending around January 7th. Knowing the due date 8+ months in advance allows time to save, arrange childcare, and coordinate work coverage.

Things to watch

•Only 5% of babies arrive on the exact due date, consider it an estimate within a 4-week window (2 weeks before to 2 weeks after).
•First-time mothers tend to deliver slightly later than their due date on average, while subsequent pregnancies may arrive a few days earlier.
•If your cycle is longer or shorter than 28 days, the LMP-based estimate may be off, an early ultrasound provides a more accurate date.
•Full term is defined as 39-40 weeks. Babies born at 37-38 weeks are "early term" and those at 41+ weeks are "late term", both require closer monitoring.
•This calculator is for informational purposes only and should not replace professional prenatal care, always confirm your due date with your healthcare provider.

Sources and methodology

Last reviewed: April 25, 2026. We review formulas, default assumptions, and examples against public references when a formal source applies.

Method: This calculator uses the formula explained on this page, then checks default assumptions and examples against the references listed below.

•Methods for Estimating the Due Date, American College of Obstetricians and Gynecologists

Found something off? Send a correction with the page URL, inputs, result, and expected result.

Common questions

How is a pregnancy due date calculated?: The standard method adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is called Naegele's rule. Only about 5% of babies arrive on their exact due date, most arrive within a two-week window around it.
How accurate are due date calculators?: LMP-based calculations are accurate to within about 2 weeks for most women. An early ultrasound (before 12 weeks) is more precise, accurate to within 3-5 days. Due dates become less reliable when menstrual cycles are irregular.
What if I do not know the date of my last period?: If you know your conception date (or approximate ovulation date), use the conception-based calculation which adds 266 days (38 weeks). If neither date is known, an early ultrasound is the most accurate method, it measures the embryo to estimate gestational age within 3-5 days during the first trimester.
What are the trimesters and when do they start?: The first trimester spans weeks 1-12 and involves early development, morning sickness, and the highest risk of miscarriage. The second trimester (weeks 13-26) is often called the easiest, energy returns and the baby grows rapidly. The third trimester (weeks 27-40) brings significant weight gain, preparation for delivery, and increasing discomfort.
Can my due date change during pregnancy?: Yes. Your healthcare provider may adjust the due date after an early ultrasound if the measurement differs from the LMP calculation by more than 7 days. Once set by ultrasound, the due date typically is not changed again. About 70% of babies are born within 10 days of their due date.