Pregnancy Due Date Calculator

A pregnancy due date calculator estimates your due date as the first day of your last menstrual period (LMP) plus 280 days (40 weeks), adjusted for cycle length. For an LMP of March 1, 2026 with a typical 28-day cycle, the estimated due date is December 6, 2026. If you know your conception date instead, add 266 days (38 weeks); after IVF, add 261 days to a day-5 transfer or 263 days to a day-3 transfer. ACOG notes that only about 1 in 20 women give birth on their due date.

This due date tool only does date math. For the health side of pregnancy planning, pair it with our BMI Calculator to check your starting weight category, the TDEE Calculator and BMR Calculator to estimate daily energy needs, and the Calorie Calculator to plan intake, always alongside your provider's guidance during pregnancy.

The formula (Naegele's rule)

The classic method counts a 40-week pregnancy from the first day of your last menstrual period (LMP):

ACOG notes that the 280-day convention assumes a regular 28-day cycle with ovulation 14 days after the period starts (cycle day 15), which is why the calculator shifts the date by the difference between your cycle length and 28 days. If you instead know your conception or ovulation date, it uses conception date + 266 days (38 weeks). For IVF, it uses transfer date + 261 days for a day-5 embryo or + 263 days for a day-3 embryo. All routes describe the same 40-week timeline from different starting points.

Due date by last period and cycle length

This matrix shows the estimated due date for several last-period dates across common cycle lengths, using Naegele's rule with the cycle-length adjustment. Every column moves the date two to seven days from the 28-day baseline.

Bold column is the standard 28-day estimate. Each day your average cycle differs from 28 moves the due date by one day.

Gestational age vs. fetal age

Gestational age is what doctors mean by "how many weeks pregnant you are." It is counted from the first day of your last period, which is why you can be "4 weeks pregnant" only about two weeks after conception. Fetal age (the baby's actual age since conception) is roughly two weeks less. ACOG divides pregnancy into three trimesters: the first runs from the first day of the LMP through 13 weeks 6 days, the second from 14 weeks 0 days through 27 weeks 6 days, and the third from 28 weeks 0 days to birth.

Why the due date is only an estimate

A due date marks 40 completed weeks, but babies rarely read the calendar. ACOG notes that only about 1 in 20 women give birth on their due date, and an NIH study of 125 pregnancies with known ovulation dates found that pregnancy length varied by 37 days even after excluding preterm births. According to the American College of Obstetricians and Gynecologists (ACOG), an ultrasound in the first trimester (up to 13 weeks 6 days) is the most accurate way to establish or confirm gestational age, and your provider may revise the LMP-based estimate after that scan. Irregular cycles, uncertain LMP dates, and assisted reproduction can all change the math.

Term definitions you'll hear

• 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, when providers typically monitor more closely.

These are the ACOG and Society for Maternal-Fetal Medicine definitions (Committee Opinion 579). Use the milestone table above to see when each phase falls on your personal timeline, then bring those dates to your prenatal visits.

How to use this calculator

You only need one date to get an estimate. Work through the fields in order:

1. Pick your method: choose last period (LMP) if you know the first day of your last menstrual period, conception if you tracked ovulation, or IVF transfer if you had an embryo transfer.
2. Enter the date: for the LMP method, use the first day of bleeding, not the last day. For the conception method, use the day you ovulated or conceived. For IVF, enter the transfer date and choose a day-3 or day-5 embryo.
3. Set your cycle length: if you used the LMP method, adjust the average cycle length away from the default 28 days. A longer cycle pushes the due date later; a shorter cycle pulls it earlier.
4. Read the results: the calculator shows your estimated due date, current gestational week and day, trimester, days remaining, and a milestone timeline. The current week is counted from today's date in your browser.

Everything updates instantly, so you can try a different cycle length or method to see how much the estimate moves. Once you have a due date, you can count the exact days to any milestone with our Date Calculator, or work out how old your baby will be on a future date with the Age Calculator.

A second worked example: counting from conception

Suppose you tracked ovulation and know you conceived on April 10, 2026. Using the conception method, the calculator adds 266 days (38 weeks) and lands on an estimated due date of about January 1, 2027. Notice that this is the same timeline you would get from an LMP two weeks earlier, March 27, 2026: the two routes describe the same 40-week pregnancy from different starting points. If today were July 1, 2026, you would be 13 weeks 5 days pregnant by gestational age, in the last days of the first trimester, which ends at 13 weeks 6 days.

Due date by conception, ovulation, and IVF method

If you know a date more precise than your last period, each method adds a different fixed number of days. This quick reference uses a known start date of March 15, 2026 so you can compare the resulting due dates side by side.

For IVF, choose the IVF transfer option, enter the transfer date and pick a day-3 or day-5 embryo; the 261- and 263-day intervals come from ACOG Committee Opinion 700.

Who this calculator is for

This tool is built for anyone who wants a quick, private estimate before or between appointments. That includes:

• People who just got a positive test and want a first idea of when the baby is due.
• Those tracking ovulation or trying to conceive, who can plan from a known conception date.
• IVF and fertility patients working from an exact transfer or retrieval date.
• Partners and family who want to follow the pregnancy week by week.
• Anyone planning ahead for parental leave, travel, or childbirth classes around a target timeline.

Key milestone dates to watch

A due date is most useful when you map the dates around it. The milestones the calculator shows, and why they matter, are:

• End of the first trimester (13 weeks 6 days): ACOG considers an ultrasound up to this point the most accurate way to date a pregnancy.
• Standard anatomy ultrasound (18 to 22 weeks): ACOG says this exam is usually done at 18 to 22 weeks to check the fetus's development; if the position allows, it may show the sex.
• Third trimester (28 weeks 0 days): the final stage, when the fetus gains most of its weight.
• Early term (37 weeks) and full term (39 weeks 0 days to 40 weeks 6 days): ACOG's term categories, used to describe when a baby is born.
• Late term (41 weeks) and post-term (42 weeks): past these points, providers usually discuss closer monitoring or induction.

Pregnancy milestone timeline by week and date

Here is the timeline worked out from a single last period of March 1, 2026 (28-day cycle, due date December 6, 2026), the same example the calculator shows before it switches to dates relative to today. Each gestational week converts to a specific calendar date you can plan around.

Factors that can shift your due date

The LMP formula assumes a textbook cycle, but real bodies vary. Several factors can move the estimate:

• Cycle length: the single biggest adjustment in this calculator; every day your average cycle differs from 28 shifts the due date by a day.
• Late or early ovulation: even with a "normal" cycle, the day you ovulate can vary month to month, which an early ultrasound captures better than any formula.
• An uncertain LMP: spotting, a light period, or recently stopping hormonal birth control can blur the start date.
• Ultrasound dating: a first-trimester scan replaces the LMP estimate if the two disagree by more than 5 days (before 9 weeks) or 7 days (9 to 13 weeks 6 days), per ACOG.
• Multiples: twins and higher-order pregnancies are dated the same way but are more likely to be born before the calculated date.

Pregnancy week by week: what each trimester brings

Knowing your due date is most useful when you can place today on the bigger arc of the pregnancy. Counting in gestational weeks from your last period, the 40 weeks fall into three broad stages:

• First trimester (first day of the LMP to 13 weeks 6 days): ACOG describes this as the time when fertilization and major organ development occur. A next period would be due at 4 weeks in a 28-day cycle, and an early ultrasound in this window is the most accurate way to confirm the due date.
• Second trimester (14 weeks 0 days to 27 weeks 6 days): a time of rapid growth and development. The standard anatomy ultrasound is usually done at 18 to 22 weeks.
• Third trimester (28 weeks 0 days to birth): the fetus's weight increases and the organs mature for life after birth. Full term begins at 39 weeks, and providers monitor more closely once you pass 40 weeks.

The progress bar and milestone dates in the calculator translate these stages into your personal calendar, so "week 20" becomes a specific date you can plan around. Because trimester boundaries are counted from the LMP, they line up exactly with the gestational age the tool reports, and with what your provider tells you at each visit, unless an early ultrasound has moved your dates.

Healthy weight, BMI, and your pregnancy timeline

A due date is the anchor for a lot of prenatal planning, and one of the things providers track against it is weight gain. Recommended gain over the full pregnancy depends on your starting point: it is generally higher if you began at a lower body mass index and lower if you began at a higher one, which is why your pre-pregnancy weight matters as much as the trimester you are in. If you want a quick sense of your starting category, our BMI Calculator estimates body mass index from height and weight, and the Weight Loss Calculator can help map a healthy plan for after delivery rather than during pregnancy. This due date tool does not factor weight into the estimate (it is purely a date calculation), but pairing the timeline with a sensible weight target is exactly the kind of conversation to have at a prenatal visit. As always, any weight-related decisions during pregnancy should be guided by your provider, not a generic calculator.

Counting backwards: planning a conception date

The same math that turns a last period into a due date also runs in reverse, which is useful if you are trying to conceive with a target birth window in mind. Because the due date is roughly the LMP plus 280 days, aiming for a baby in a particular month means conceiving about 38 weeks earlier, or starting a cycle about 40 weeks earlier. For example, a target due date in early September points to conception in early-to-mid December and a last period in late November. The catch is that ovulation timing varies and conception may take several cycles, so reverse planning gives a window rather than a guarantee. The Reverse Due Date Calculator runs this math for any target date. Once you do have a positive test, switch to the LMP or conception method above for the actual estimate, since a real start date is far more reliable than a planned one.

Limitations and assumptions

This calculator is an estimate for general planning, not a medical diagnosis. Keep these assumptions in mind:

• It assumes a single pregnancy and does not adjust for twins or multiples, which are more likely to arrive early.
• It cannot account for medical conditions or pregnancy complications that may change your delivery date.
• The LMP method assumes regular cycles and a reliably known last period; both can be uncertain in practice.
• It does not replace ultrasound dating, which is the most accurate method in the first trimester.
• The result is the midpoint of a range: ACOG counts births from 37 weeks 0 days through 41 weeks 6 days as term.

How it compares to ultrasound dating

A calculator and an ultrasound answer the same question with different tools. The LMP method is free, instant, and works from a date you already know, which makes it perfect for an early estimate. An early ultrasound actually measures the embryo (the crown-rump length), and because early growth is remarkably consistent from pregnancy to pregnancy, it is the most accurate dating method in the first trimester. The two usually agree closely; when a first-trimester scan differs by more than 5 to 7 days, ACOG guidance is to use the ultrasound date. Think of this calculator as your starting point and the ultrasound as the confirmation; they work best together.

Sources

• American College of Obstetricians and Gynecologists (ACOG): Committee Opinion 700, Methods for Estimating the Due Date (reaffirmed 2025: LMP + 280 days, ultrasound redating thresholds, IVF + 261 or 263 days).
• ACOG: Committee Opinion 579, Definition of Term Pregnancy (early, full, late and post-term).
• ACOG: How Your Fetus Grows During Pregnancy (trimester definitions; about 1 in 20 births on the due date) and Ultrasound Exams (standard exam at 18 to 22 weeks).
• Jukic AM, et al. (2013): Length of human pregnancy and contributors to its natural variation, Human Reproduction (NIH study).
• Office on Women's Health, U.S. Department of Health and Human Services: Pregnancy.
• Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), NIH: Pregnancy.