How is a due date calculated?

A due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period using Naegele's Rule. If your cycle is longer or shorter than 28 days, the date adjusts accordingly. For example, a 32-day cycle shifts the due date 4 days later. If you know your conception date, add 266 days instead. ACOG recommends confirming dates with a first-trimester ultrasound, which is accurate to within 5-7 days.

How accurate is a due date calculator?

Only about 5% of babies are born on their exact estimated due date, according to research published in the BMJ. Most births occur within a 2-week window around the due date. An early ultrasound (before 12 weeks) provides the most accurate dating. Natural pregnancy length varies by up to 37 days between individuals, so the due date should be considered an estimate rather than a precise prediction.

What are the three trimesters of pregnancy?

The first trimester spans weeks 1-12 and includes organ formation. The second trimester covers weeks 13-27 and is when most women feel their best. The third trimester runs from week 28 to delivery around week 40. Key milestones include the first heartbeat (week 6), nuchal translucency scan (week 12), anatomy scan (week 20), viability (week 24), and full term (week 37-40).

Does cycle length affect the due date?

Yes, cycle length directly affects the estimated due date. A longer cycle means ovulation occurs later, so the due date shifts forward by the same number of days beyond 28. For example, a woman with a 35-day cycle ovulates around day 21 instead of day 14, so her due date is 7 days later than the standard calculation. A shorter 24-day cycle moves the due date 4 days earlier.

When should I see a doctor after a positive pregnancy test?

ACOG recommends scheduling your first prenatal visit within the first 8-10 weeks of pregnancy. Early prenatal care allows for dating ultrasound, blood work, and identification of any risk factors. Contact your provider sooner if you experience severe nausea, bleeding, or pain. Most practitioners schedule the first appointment around weeks 8-10 from your LMP.

What is the difference between LMP-based and conception-based dating?

LMP-based dating adds 280 days from the first day of your last period and is the standard clinical method because most women know their LMP date. Conception-based dating adds 266 days from the actual fertilization date and can be more precise for women who tracked ovulation. The difference is approximately 14 days. For IVF pregnancies, conception-based dating is preferred because the exact fertilization date is known.

Due Date Calculator

Quick Answer

Your estimated due date is 280 days (40 weeks) from the first day of your last menstrual period, per Naegele's Rule used by the American College of Obstetricians and Gynecologists (ACOG). If you know the conception date, add 266 days. Only about 4% of babies are born on the exact due date; most arrive within two weeks of it.

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Calculate From

Last Period Start Date

Average Cycle Length (days)

Estimated Due Date

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Current Week (if applicable)

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Conception Window

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Trimester Milestones

Milestone	Week	Estimated Date

How a Due Date Calculator Works

A due date calculator is a tool that estimates when a baby is expected to be born based on the date of the mother's last menstrual period (LMP) or conception date. The standard method used by obstetricians worldwide is Naegele's Rule, which adds 280 days (40 weeks) to the first day of the LMP, according to the American College of Obstetricians and Gynecologists (ACOG).

This calculator supports two calculation modes. When using the LMP method, it adds 280 days and adjusts for cycle length: if your cycle is 32 days instead of 28, ovulation likely occurred 4 days later, so the due date shifts 4 days forward. When using a known conception date, the calculator adds 266 days (38 weeks), since conception typically happens about 14 days after the LMP. The tool also generates a trimester milestone timeline so you can track key appointments and developmental stages.

According to ACOG, only about 5% of babies are born on their exact due date, and the normal range for delivery spans from 37 weeks (early term) to 42 weeks (late term). First-time mothers tend to deliver slightly later than experienced mothers. An early ultrasound before 12 weeks can refine the estimated due date to within 5-7 days of accuracy. You can also use our Ovulation Calculator to estimate when conception may have occurred, or the Pregnancy Calculator for a week-by-week breakdown.

The Due Date Formula (Naegele's Rule)

Naegele's Rule is the standard formula for estimating a pregnancy due date. It was developed by German obstetrician Franz Naegele in the early 19th century and remains the most widely used method in clinical practice today, as referenced by the National Institute of Child Health and Human Development (NICHD).

From LMP: Due Date = LMP + 280 days + (Cycle Length - 28)

From Conception: Due Date = Conception Date + 266 days

Worked example: If your LMP was January 1, 2026, and your cycle is 30 days: Due Date = January 1 + 280 + 2 = October 10, 2026. The 2-day adjustment accounts for later ovulation in a 30-day cycle compared to a standard 28-day cycle.

Key Pregnancy Terms You Should Know

Last Menstrual Period (LMP): The first day of your most recent menstrual period, used as the starting point for gestational age.
Gestational Age: The age of the pregnancy measured from the LMP, typically expressed in weeks and days.
Estimated Due Date (EDD): The predicted date of delivery, also called the expected date of confinement (EDC).
Trimester: Pregnancy is divided into three trimesters: first (weeks 1-12), second (weeks 13-27), and third (weeks 28-40).
Full Term: A pregnancy that reaches 39-40 weeks. Early term is 37-38 weeks; late term is 41 weeks; post-term is 42+ weeks.
Viability: The gestational age at which a fetus can survive outside the womb, generally considered around 24 weeks.

Pregnancy Timeline: Week-by-Week Milestones

Understanding key milestones helps expectant parents prepare for appointments and track fetal development. The following table summarizes the most important dates during pregnancy, based on ACOG pregnancy guidelines.

Week	Milestone	Details
Week 6	First heartbeat	Fetal heartbeat may be detected via transvaginal ultrasound
Week 12	End of 1st trimester	Nuchal translucency scan; miscarriage risk drops significantly
Week 20	Anatomy scan	Detailed ultrasound to check fetal organs and structures
Week 24	Viability threshold	Survival outside the womb becomes possible with NICU care
Week 28	End of 2nd trimester	Glucose tolerance test; Rh factor testing
Week 37	Early term	Baby is considered early term; lungs typically mature
Week 39-40	Full term / Due date	Optimal delivery window; lowest risk of complications

Practical Examples

Example 1 - Standard 28-day cycle: Sarah's LMP was March 1, 2026. Using Naegele's Rule: March 1 + 280 days = December 6, 2026. Her first trimester ends around May 24, the anatomy scan is around July 19, and she reaches full term around November 15.

Example 2 - Longer cycle: Emma has a 35-day cycle with LMP on February 1, 2026. Adjustment: 280 + (35 - 28) = 287 days. Due date: November 15, 2026, which is 7 days later than a 28-day cycle would predict. This shift is important because ovulation occurs around day 21 in a 35-day cycle rather than day 14.

Example 3 - From conception date: Lisa knows she conceived on April 15, 2026 (confirmed by ovulation tracking with our Ovulation Calculator). Due date: April 15 + 266 = January 6, 2027. She can track her pregnancy weight gain throughout each trimester.

Tips for a Healthy Pregnancy

Schedule early prenatal care: Book your first appointment as soon as you get a positive test. Early ultrasound (before 12 weeks) provides the most accurate dating, according to ACOG.
Track your cycle length: Knowing your average cycle length improves due date accuracy. Use a Period Calculator to identify patterns over several months.
Prepare for a range, not a date: Since only 5% of deliveries happen on the exact due date, plan for delivery anywhere from 37-42 weeks. Have your hospital bag packed by week 36.
Monitor weight gain: The Institute of Medicine recommends 25-35 pounds of total weight gain for women with a normal pre-pregnancy BMI. Track this with our Pregnancy Weight Gain Calculator.
Take prenatal vitamins: The CDC recommends 400 mcg of folic acid daily, starting at least one month before conception and continuing through pregnancy to reduce neural tube defect risk.

Due Date Accuracy: What the Research Shows

According to a large study published in the BMJ, the average length of a human pregnancy is 268 days from ovulation (approximately 282 days from LMP), with a natural variation of up to 37 days. The World Health Organization defines preterm birth as delivery before 37 completed weeks, affecting approximately 10% of births worldwide. Full-term delivery at 39-40 weeks is associated with the lowest rates of neonatal complications. First-time mothers deliver an average of 5 days past their due date, while subsequent pregnancies tend to be slightly shorter.

Disclaimer: This calculator is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare provider for decisions specific to your pregnancy.

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