Pregnancy Due Date Calculator

Say the first day of your last menstrual period was March 1, 2026 and you have a typical 28-day cycle. Using Naegele's rule, you add 280 days (40 weeks) and land on an estimated due date of about December 6, 2026. If your cycle runs longer - say 32 days - this pregnancy due date calculator nudges the date four days later, to around December 10, because you likely ovulate later in a longer cycle. That single adjustment is one of the biggest reasons online estimates differ.

The formula (Naegele's rule)

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

If you instead know your conception or ovulation date, the calculator uses conception date + 266 days (38 weeks), since conception happens about two weeks after the LMP. Both routes are estimates of the same 40-week timeline measured from slightly different starting points.

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. Pregnancy is divided into three trimesters: the first runs through week 13, the second through week 27, and the third from week 28 to birth.

Why the due date is only an estimate

A due date marks 40 completed weeks, but babies rarely read the calendar. Only about 1 in 20 are born on the exact estimated date; most arrive within two weeks on either side. According to the American College of Obstetricians and Gynecologists (ACOG), an early ultrasound in the first trimester is the most accurate way to date a pregnancy, 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 - the ideal window.
• 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.

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, or conception date if you tracked ovulation or had an IVF 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, conceived, or had your transfer.
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.

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 roughly two weeks earlier, around 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 about 13 weeks 4 days pregnant by gestational age, placing you right at the start of the second trimester, just past the end-of-first-trimester milestone.

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 - 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 surfaces - and why they matter - are:

• End of the first trimester (week 13): the risk of miscarriage drops sharply, and many people share their news around now.
• Anatomy scan (about weeks 18-22): a detailed ultrasound that checks the baby's development and can often reveal the sex.
• Viability (about week 24): the point at which a baby has a meaningful chance of survival with intensive care if born early.
• Third trimester (week 28): prenatal visits become more frequent and you begin preparing for delivery.
• Full term (39-40 weeks): the ideal window for birth, with the lowest risk of complications.
• Post-term (42 weeks): if you pass this point, providers usually discuss monitoring or induction.

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 can override the LMP estimate if the two disagree by more than about a week.
• Multiples: twins and higher-order pregnancies are dated the same way but very often deliver earlier than 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 (weeks 1-13): the period from the last menstrual period through implantation and early organ formation. Many people first see a positive test around weeks 4-5 and have a dating or viability scan between weeks 6 and 9. Symptoms such as nausea and fatigue are common, and the trimester ends with the risk of miscarriage dropping sharply by week 13.
• Second trimester (weeks 14-27): often called the "honeymoon" stage because early symptoms ease. The anatomy scan around weeks 18-22 checks the baby's development and can reveal the sex, and most people feel the first movements ("quickening") somewhere between weeks 16 and 22.
• Third trimester (weeks 28-40+): the baby gains most of its weight, prenatal visits become more frequent, and viability outside the womb improves steadily. 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, so reverse planning gives a window rather than a guarantee - most people who are actively trying conceive within several cycles, not on a single chosen 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 commonly 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 a midpoint of a normal range - a healthy birth can happen any time from about 37 to 42 weeks.

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 they differ by more than about a week, providers trust the scan. 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) - How Your Fetus Grows During Pregnancy.
• Office on Women's Health, U.S. Department of Health & Human Services - Pregnancy.
• Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), NIH - Pregnancy.
• MedlinePlus, U.S. National Library of Medicine (NIH) - Pregnancy.