Pregnancy Due Date Calculator
Enter the first day of your last menstrual period and your typical cycle length to estimate your due date, current gestational week, trimester, and conception date.
How Due Date Calculation Works
Pregnancy due dates trace back to a formula developed by Franz Naegele, a German obstetrician practicing in the early 19th century. His method was remarkably simple: take the first day of the woman's last menstrual period, add seven days, subtract three months, then add one year. That arithmetic lands on a date exactly 280 days — or 40 weeks — from the start of the last period.
The reason the calculation starts from the last period rather than conception is practical. Most women can recall when their period started with reasonable accuracy. Pinpointing the exact moment of conception is far harder, since sperm can survive inside the reproductive tract for up to five days and ovulation timing varies. Using the LMP as a starting reference gave doctors a consistent anchor point, even though the baby didn't actually exist for the first two weeks of that 40-week count.
This calculator applies an important correction that basic Naegele's rule skips: cycle length adjustment. The standard formula assumes every woman ovulates on day 14 of a 28-day cycle. But cycles naturally range from about 21 to 35 days in healthy adults. A woman with a 35-day cycle probably ovulates closer to day 21, which means conception happens roughly a week later than the standard model predicts. Without adjusting for this, her due date would be calculated a week too early.
The adjustment is straightforward. Subtract 28 from the actual cycle length, and add that number of days to the standard 280-day estimate. A 32-day cycle adds 4 days. A 24-day cycle subtracts 4 days. It's a simple correction that meaningfully improves accuracy for women whose cycles don't match the textbook 28-day template.
Understanding the Three Trimesters
Pregnancy is divided into three trimesters, each spanning roughly 13 weeks. The divisions aren't arbitrary — they correspond to distinct phases of fetal development that carry different medical considerations and physical experiences for the mother.
The first trimester covers weeks 1 through 12. During this period, the fertilized egg implants in the uterine wall, and the embryo undergoes an astonishing transformation from a cluster of cells into a recognizable human form. By week 8, all major organs have begun forming. The heart starts beating around week 5 or 6 — often before the mother even realizes she's pregnant. This trimester carries the highest risk of miscarriage, with about 80% of pregnancy losses occurring before week 12. Morning sickness, fatigue, and breast tenderness are common symptoms as hormone levels surge.
The second trimester spans weeks 13 through 27 and is often described as the most comfortable stretch. Nausea typically fades, energy returns, and the risk of miscarriage drops sharply. The baby grows from about 3 inches to 14 inches in length. Around week 18 to 22, most women feel the first fetal movements — a milestone called quickening. The anatomy scan, usually performed between weeks 18 and 20, provides a detailed ultrasound assessment of the baby's organs and can reveal the sex if the parents want to know.
The third trimester runs from week 28 to delivery, usually around week 40. The baby gains significant weight during this phase — often doubling or tripling in size. Lungs mature, the brain develops rapidly, and the baby shifts into a head-down position in preparation for birth. For the mother, this trimester brings increasing physical demands: back pain, swollen feet, frequent urination, and difficulty sleeping are all par for the course. Braxton Hicks contractions — practice contractions that don't signal actual labor — become more common in the final weeks.
How Accurate Are Due Date Predictions?
Here is something that surprises many expectant parents: only about 4% to 5% of babies arrive on their actual due date. The 280-day figure is a statistical average, not a biological deadline. Full-term pregnancy spans a range from 37 weeks to 42 weeks, and healthy babies routinely arrive anywhere within that window.
A 2013 study published in Human Reproduction tracked 125 pregnancies where the exact date of ovulation was confirmed through hormone monitoring. The researchers found that the natural length of pregnancy varied by as much as 37 days across the group. Some pregnancies lasted 247 days from ovulation. Others went to 284 days. Both endpoints produced healthy deliveries. The variation was striking, and it wasn't explained by the mother's age, BMI, or prior pregnancy history.
Ultrasound dating, particularly when performed in the first trimester, is generally more accurate than LMP-based calculations. A first-trimester ultrasound can estimate gestational age within about 5 to 7 days. By the second trimester, that margin widens to 10 to 14 days, because individual growth rates start to diverge. Doctors often adjust the due date if an early ultrasound disagrees with the LMP calculation by more than 7 days.
First-time mothers tend to deliver slightly later than their due date — about 5 days past on average. Women who have given birth before tend to deliver a bit earlier. Neither pattern is a rule, just a statistical tendency. The bottom line is that a due date is best understood as the center of a two-to-three-week window rather than a specific target date circled on the calendar.
Key Prenatal Milestones and What to Expect
Pregnancy involves a long sequence of medical appointments, tests, and developmental markers. Knowing what's ahead helps parents plan and reduces the anxiety that comes from uncertainty.
The first prenatal visit typically happens between weeks 8 and 10. The doctor confirms the pregnancy, estimates the due date, runs blood tests to check blood type, Rh factor, iron levels, and screens for infections like HIV, hepatitis B, and syphilis. A urine test checks kidney function and screens for gestational diabetes risk factors. This appointment sets the baseline for everything that follows.
Around week 10 to 13, many parents opt for the NIPT (non-invasive prenatal test), a blood draw that screens for chromosomal conditions like Down syndrome, trisomy 18, and trisomy 13. The test analyzes fragments of fetal DNA circulating in the mother's blood and has a detection rate above 99% for the most common conditions. It can also reveal the baby's sex with high accuracy.
The anatomy scan at weeks 18 to 20 is one of the most anticipated appointments. A detailed ultrasound examines the baby's brain, heart, spine, kidneys, limbs, and other structures. It measures growth, checks the amount of amniotic fluid, and evaluates the placenta's position. Many parents see this as the first real look at their baby.
Glucose screening at weeks 24 to 28 tests for gestational diabetes, a condition that affects roughly 6% to 9% of pregnancies. The mother drinks a sugary solution and has blood drawn an hour later to measure how well her body processes the glucose. If levels are elevated, a longer three-hour test follows to confirm the diagnosis.
Starting around week 36, prenatal visits increase to weekly. The doctor monitors the baby's position, checks for signs of preeclampsia (high blood pressure during pregnancy), and assesses cervical changes that indicate labor may be approaching. A Group B strep test, performed between weeks 35 and 37, determines whether the mother carries a bacterium that could pose risks during vaginal delivery.
Naegele's Rule (Adjusted)
Due Date = LMP + 280 days + (Cycle Length - 28)
Naegele's rule has been the standard method for estimating due dates since the early 1800s. The basic version adds 280 days (40 weeks) to the first day of the last menstrual period. This assumes a 28-day cycle with ovulation occurring on day 14. For women with longer or shorter cycles, an adjustment is applied: add or subtract the difference between the actual cycle length and 28 days. A woman with a 32-day cycle would have her due date shifted 4 days later, since ovulation likely occurs around day 18 instead of day 14.
Where:
- LMP = First day of the last menstrual period
- 280 days = Standard gestational period (40 weeks) from LMP
- Cycle Length - 28 = Adjustment in days for cycles that differ from the assumed 28-day standard
- Conception = Estimated at LMP + 14 days (adjusted for cycle length)
Example Calculations
Standard 28-Day Cycle
Calculating the due date for a woman with a regular 28-day cycle whose last period started on January 15, 2026.
Using Naegele's rule: January 15 + 280 days = October 22, 2026. Since the cycle length is exactly 28 days, no adjustment is needed. Estimated conception date is LMP + 14 days = January 29. As of March 15, 2026, the pregnancy is at 8 weeks and 4 days (59 days from LMP), placing it in the first trimester.
Longer 33-Day Cycle
A woman with a 33-day cycle whose last period began on December 1, 2025.
Standard calculation: December 1 + 280 days = September 6, 2026. Cycle adjustment: 33 - 28 = 5 extra days, so the adjusted due date is September 11, 2026. Estimated conception: LMP + 14 + (33 - 28) = December 1 + 19 = December 20. As of March 15, 2026, she is 105 days from LMP, which is 15 weeks and 0 days, placing her in the second trimester.
Frequently Asked Questions
Gestational age has been counted from the last menstrual period for over 200 years because it provides a reliable, easily identifiable starting date. Most women can remember when their last period began, but very few can identify the exact date of conception. Ovulation can shift by several days from cycle to cycle, and sperm can survive in the reproductive tract for up to five days, making the moment of fertilization nearly impossible to pinpoint without laboratory monitoring. The medical community standardized on LMP dating for consistency across clinical practice, research, and communication between healthcare providers.
If you're unsure, using 28 days is a reasonable default since it's the statistical average for adult women. However, if you have a general sense that your cycles run longer or shorter — say you typically get your period every 31 to 33 days — using that estimate will produce a more accurate due date than defaulting to 28. Cycle tracking apps on your phone can help establish your pattern over a few months. If your cycles are very irregular, varying by more than 7 to 8 days from month to month, an early ultrasound will give the most reliable dating.
Yes, and it frequently does. A first-trimester ultrasound measures the embryo from crown to rump, and that measurement is highly correlated with gestational age during the early weeks. If the ultrasound date differs from the LMP-based estimate by more than 7 days, most practitioners will adjust the official due date to match the ultrasound. After the first trimester, ultrasound dating becomes less precise because babies start growing at different rates. A second-trimester scan might show a baby measuring large or small for gestational age without necessarily meaning the due date is wrong.
Absolutely. A pregnancy is considered full term between 39 weeks and 40 weeks 6 days, early term between 37 and 38 weeks 6 days, and late term at 41 weeks through 41 weeks 6 days. Delivery anywhere in the 37-to-42-week window is within the normal range. Babies born before 37 weeks are classified as preterm, and those arriving after 42 weeks are post-term. Most obstetric guidelines recommend induction or close monitoring if pregnancy extends past 41 to 42 weeks, since risks to the baby begin to increase beyond that point.
For IVF pregnancies, the due date calculation is slightly different because the date of embryo transfer is known precisely. Doctors typically calculate the due date based on the transfer date rather than a last menstrual period. For a day-5 blastocyst transfer, the formula is transfer date + 261 days (or equivalently, transfer date minus 19 days gives an equivalent LMP date, then add 280 days). This calculator uses LMP-based dating, so for IVF you would need to work backward from the transfer date to find your equivalent LMP. Your fertility clinic will provide the most accurate due date for your specific situation.