How to Calculate Your Due Date: A Complete Pregnancy Guide
A pregnancy due date(estimated date of delivery, or EDD) is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This method, called Naegele's rule, has been the standard in obstetrics since the early 1800s. About 3.6 million babies are born in the United States each year (CDC, 2023), and every one of them started with this same calculation.
Quick Answer
- 1. Standard due date formula: first day of LMP + 280 days (Naegele's rule).
- 2. Only 5% of babies are born on their exact due date (ACOG, 2023). Most arrive within a 2-week window.
- 3. First-trimester ultrasound is accurate to plus or minus 5-7 days and is the gold standard for dating (ACOG, 2022).
- 4. First-time mothers deliver on average 5 days past their due date (BMJ, 2023).
Calculate your estimated due date
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Pregnancy Calculator - FreeHow Due Dates Are Calculated
The standard method for calculating a pregnancy due date is Naegele's rule, named after German obstetrician Franz Naegele, who published it in 1812. The formula is straightforward: take the first day of your last menstrual period, add one year, subtract three months, and add seven days. In practice, this works out to adding 280 days (40 weeks) to the LMP date.
This 280-day figure assumes a 28-day menstrual cycle with ovulation occurring on day 14. Since pregnancy is dated from the LMP rather than the actual date of conception, the first two weeks of "pregnancy" are technically before the egg was even fertilized. Actual embryonic age is approximately two weeks less than gestational age at any point during pregnancy.
For women with cycles longer or shorter than 28 days, the calculation adjusts accordingly. If your cycle is 32 days, ovulation likely occurred around day 18 rather than day 14, so your due date would shift four days later than the standard Naegele calculation. Many clinicians use a modified version of the formula that accounts for individual cycle length.
Ultrasound Dating vs. LMP Dating
While LMP-based dating is the starting point for nearly every pregnancy, ultrasound measurement is considered more accurate, particularly in early pregnancy. The American College of Obstetricians and Gynecologists published guidelines on pregnancy dating accuracy:
- First trimester (up to 13 weeks 6 days): Ultrasound is accurate to within 5-7 days. Crown-rump length (CRL) is the primary measurement used.
- Second trimester (14-27 weeks): Accuracy drops to plus or minus 10-14 days. Head circumference, femur length, and abdominal circumference are measured.
- Third trimester (28+ weeks): Accuracy is plus or minus 21 days, making ultrasound unreliable for initial dating at this stage.
When first-trimester ultrasound dating disagrees with LMP dating by more than 7 days, ACOG recommends using the ultrasound-derived date. This matters because accurate dating affects the timing of prenatal screening tests, determines when a pregnancy is considered full-term versus preterm, and guides decisions about labor induction.
Trimester Breakdown
Pregnancy is divided into three trimesters, each lasting approximately 13-14 weeks. Here is what happens during each stage:
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1-12 | All major organs form. Heart begins beating (week 6). Neural tube closes. Highest risk of miscarriage. Morning sickness peaks weeks 8-10. |
| Second | 13-27 | Rapid growth. Sex can be determined (week 18-20). Mother feels first movement (weeks 16-25). Anatomy scan at week 20. Viability threshold around week 24. |
| Third | 28-40 | Lungs mature. Baby gains most of birth weight. Brain development accelerates. Baby moves into head-down position. Full term at 39 weeks. |
Why Only 5% of Babies Arrive on Their Due Date
The 280-day estimate is a statistical average, not a precise prediction. A due date marks the midpoint of a normal delivery window, not a deadline. According to ACOG, full-term pregnancy spans from 39 weeks 0 days through 40 weeks 6 days, but healthy deliveries can occur anywhere from 37 weeks (early term) through 41 weeks 6 days (late term).
Research published in BMJ found that first-time mothers deliver on average 5 days past their due date. Second-time mothers tend to deliver closer to the due date or slightly before. The wide variation exists because gestational length is influenced by genetics, maternal age, parity (number of previous pregnancies), and the baby's sex. Male babies tend to have slightly longer gestations than female babies.
The practical takeaway: think of your due date as the center of a 4-week window (38-42 weeks) rather than a specific day.
Key Prenatal Milestones by Week
The following table summarizes the major prenatal appointments, tests, and fetal developments that most pregnancies share:
| Week | Milestone |
|---|---|
| 6-8 | First prenatal visit. Confirm pregnancy via ultrasound. Hear heartbeat. Blood work and urine tests. |
| 10-13 | First-trimester screening (nuchal translucency ultrasound + blood test for chromosomal conditions). |
| 11-14 | Cell-free DNA screening (NIPT) available for chromosomal abnormalities. |
| 16 | Second-trimester blood screening (quad screen). Some mothers feel first fetal movement. |
| 18-20 | Anatomy scan (detailed ultrasound). Sex determination. Check organ development. |
| 24-28 | Glucose tolerance test for gestational diabetes. Viability threshold reached (~24 weeks). |
| 28 | Rh antibody screen if Rh-negative. RhoGAM injection if needed. Begin third-trimester visits (every 2 weeks). |
| 32 | Baby is in approximate birth position. Growth ultrasound if indicated. |
| 36 | Group B Strep (GBS) test. Weekly visits begin. Check cervical changes. |
| 37 | Early term. Baby's lungs are likely mature. |
| 39-40 | Full term. Ideal delivery window. Discuss induction plans if pregnancy extends past 41 weeks. |
Factors That Affect Due Date Accuracy
Several factors can cause your actual delivery date to differ from the calculated due date:
- Irregular menstrual cycles: Women with cycles shorter than 25 days or longer than 35 days are more likely to have inaccurate LMP-based dates. Irregular ovulation shifts the true conception date away from the assumed day-14 model.
- Late or no first-trimester ultrasound: Without early ultrasound confirmation, dating relies entirely on LMP recall, which studies show is inaccurate in up to 40% of pregnancies.
- IVF or assisted reproduction: Pregnancies conceived through IVF have the most accurate dating because the embryo transfer date is known precisely. Due dates are calculated from the transfer date rather than LMP.
- Maternal age: Women over 35 have a slightly higher rate of both preterm delivery and post-term pregnancy.
- Multiple pregnancy: Twins deliver at a median of 36 weeks, and triplets at approximately 32 weeks, well before the 40-week due date.
- Preterm birth risk: About 11% of all births in the US are preterm (before 37 weeks), according to CDC 2023 data. Risk factors include prior preterm birth, short cervix, infection, and multiple gestation.
See your full pregnancy timeline
Our free pregnancy calculator shows your due date, current week, trimester, and upcoming milestones.
Pregnancy Calculator - FreeWhen to See a Doctor: Key Appointment Schedule
ACOG recommends the following prenatal visit schedule for uncomplicated pregnancies:
- Weeks 4-28: One visit every 4 weeks (approximately 7 visits).
- Weeks 28-36: One visit every 2 weeks (approximately 4 visits).
- Weeks 36-delivery: One visit per week (approximately 4 visits).
This adds up to roughly 12-15 prenatal visits total. High-risk pregnancies (maternal age over 35, multiples, gestational diabetes, preeclampsia history, or chronic conditions) may require more frequent monitoring, additional ultrasounds, and specialist consultations.
Contact your healthcare provider immediately if you experience heavy bleeding, severe abdominal pain, sudden severe headache with vision changes, significant decrease in fetal movement after 28 weeks, or signs of preterm labor (regular contractions before 37 weeks, pressure in the pelvis, or fluid leaking from the vagina).
The Bottom Line
Your pregnancy due date is an estimate based on a 280-day average, not a guarantee. It gives you and your healthcare team a framework for scheduling prenatal care, screening tests, and delivery planning. The most accurate dating comes from combining your LMP with a first-trimester ultrasound. Remember that only 5% of babies arrive on the exact due date, and a healthy delivery can happen anywhere in the 37-to-42-week window.
Calculate your estimated due date and see a week-by-week pregnancy timeline with our free pregnancy calculator.
Frequently Asked Questions
How is a pregnancy due date calculated?
The most common method is Naegele's rule: take the first day of your last menstrual period (LMP), add 280 days (40 weeks), and that is your estimated due date. This assumes a 28-day menstrual cycle with ovulation on day 14. For women with longer or shorter cycles, the calculation is adjusted. For example, if your cycle is 35 days, your due date shifts about one week later than the standard LMP calculation. Ultrasound measurements in the first trimester can also be used to date the pregnancy, and are considered more accurate than LMP dating when there is a discrepancy of more than 7 days.
Why do doctors count pregnancy from the last menstrual period instead of conception?
Doctors use the LMP because most women know when their last period started, but few know the exact date of conception. Ovulation and fertilization typically occur about two weeks after the LMP in a 28-day cycle, which means the embryo is actually about two weeks younger than the gestational age. This convention has been used in obstetrics for over 200 years and is the basis for all standard pregnancy timelines, trimester divisions, and prenatal care schedules.
How accurate is an ultrasound due date compared to LMP?
First-trimester ultrasound (before 14 weeks) is the most accurate dating method, with a margin of error of plus or minus 5 to 7 days according to the American College of Obstetricians and Gynecologists. Second-trimester ultrasound (14-28 weeks) is accurate to plus or minus 10 to 14 days. Third-trimester ultrasound is the least reliable for dating, with a margin of plus or minus 21 days. When ultrasound dating and LMP dating differ by more than 7 days in the first trimester, clinicians typically use the ultrasound date.
What percentage of babies are born on their due date?
Only about 5% of babies are born on their exact estimated due date, according to ACOG. Most births occur within a two-week window around the due date. A study published in BMJ found that first-time mothers deliver on average 5 days past their due date. Full-term delivery is defined as 39 weeks 0 days through 40 weeks 6 days. Babies born between 37 and 38 weeks 6 days are considered early term, and those born at 41 weeks 0 days through 41 weeks 6 days are considered late term.
What are the three trimesters and when does each start?
The first trimester spans weeks 1 through 12 (LMP dating), during which all major organs form and the risk of miscarriage is highest. The second trimester covers weeks 13 through 27, often called the most comfortable period, when the baby grows rapidly and most women feel fetal movement for the first time (typically between weeks 16 and 25). The third trimester runs from week 28 until delivery, during which the baby gains most of its birth weight and the lungs mature. Each trimester lasts approximately 13 to 14 weeks.
Can my due date change during pregnancy?
Yes. Your due date may be adjusted after an early ultrasound if the measurements differ significantly from the LMP-based estimate. ACOG guidelines state that if first-trimester ultrasound dating differs from LMP dating by more than 7 days, the ultrasound date should be used. In the second trimester, the threshold is 10 to 14 days. Once a due date is established by early ultrasound, it generally should not be changed by subsequent ultrasounds, as later measurements become less accurate for dating purposes.
What factors can make a baby come earlier or later than the due date?
Factors associated with earlier delivery include carrying multiples (twins arrive at a median of 36 weeks), preeclampsia, gestational diabetes requiring intervention, cervical insufficiency, and a history of preterm birth. Factors associated with later delivery include being a first-time mother (primiparas average 5 days past the due date), having a family history of post-term pregnancies, and carrying a male baby (which statistically correlates with slightly longer gestation). About 11% of births in the US are preterm, occurring before 37 weeks, according to the CDC.
Calculate your pregnancy due date
Enter the first day of your last period to see your estimated due date, trimester progress, and key milestones.
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