A pregnancy due date calculator estimates the day your baby will arrive. Enter one key date, and the calculator returns your estimated due date along with how far along you are right now.
Most pregnancies last about 40 weeks from the first day of your last menstrual period. That is roughly 280 days. This calculator uses that standard, plus adjustments for your specific inputs, to find your estimated due date in seconds.
You can calculate your due date using any of these starting points:
- First day of your last menstrual period (LMP)
- Date of conception or ovulation
- IVF embryo transfer date
- Ultrasound scan measurements
Every pregnancy is unique. The date this calculator returns is an estimate, not a guarantee. Your healthcare provider will confirm or adjust your due date based on prenatal exams and ultrasound results.
How the Due Date Calculator Works
The calculator takes a single date you provide, applies a standard formula, and returns your estimated due date. Which formula it uses depends on the method you choose.
Estimate based on last period
This is the most common method. The calculator adds 280 days (40 weeks) to the first day of your last menstrual period. If your cycle length differs from the standard 28 days, it adjusts the result accordingly.
You do not need to know when you ovulated. The formula assumes ovulation happened about 14 days before your next period would have started.
Estimate based on conception date
If you know (or can closely estimate) the date of conception, the calculator adds 266 days from that date. This skips the roughly two weeks between your period and ovulation that the LMP method includes.
Estimate based on IVF transfer date
For IVF pregnancies, the transfer date and embryo age are known precisely. The calculator adjusts the formula based on whether a Day 3 or Day 5 embryo was transferred.
- Day 3 transfer: adds 263 days from the transfer date
- Day 5 transfer: adds 261 days from the transfer date
Estimate based on ultrasound scan
An early ultrasound measures the embryo's size, which correlates closely with gestational age. If you enter the gestational age from your scan along with the scan date, the calculator projects forward to 40 weeks.
Ultrasound is often more precise than LMP, especially when cycle length is uncertain or periods are irregular.
Estimated Due Date and Gestational Age
Your estimated due date and gestational age are connected but describe different things. The due date is a single calendar day. Gestational age is how far along your pregnancy is right now, counted in weeks and days.
How gestational age relates to your due dates
Gestational age starts counting from the first day of your last menstrual period. At your due date, gestational age equals exactly 40 weeks and 0 days.
When a provider says you are "12 weeks pregnant," that is your gestational age. It tells you where you are on the pregnancy timeline and helps your care team track development milestones.
The difference between gestational age and fetal (conceptional) age is about two weeks. Fetal age counts from the actual date of conception. Gestational age counts from your LMP, which is roughly two weeks earlier. Medical records and prenatal care almost always use gestational age.
Why pregnancy is counted from your last menstrual period
Conception usually happens around day 14 of a menstrual cycle. But most people do not know the exact date of conception. The first day of the last period is a date nearly everyone can recall or confirm.
Counting from the LMP gives providers a consistent, reliable starting point. It has been the medical standard for decades, and all pregnancy milestones, trimester boundaries, and growth charts are built around it.
This means you are technically "pregnant" for about two weeks before fertilization actually occurs. It sounds odd, but it keeps the math consistent across every pregnancy.
Calculate Your Due Date by Last Period
To use this method, enter the first day of your last menstrual period and your average cycle length. The calculator handles the rest.
The basic formula (called Naegele's Rule) works like this:
- Start with the first day of your last period
- Add 280 days (40 weeks)
- Adjust if your cycle is longer or shorter than 28 days
If your cycle is 32 days, for example, the calculator adds 4 extra days. If your cycle is 25 days, it subtracts 3 days. The adjustment assumes ovulation shifts by roughly the same number of days your cycle differs from 28.
Why cycle length matters for estimating the due date
A standard 28-day menstrual cycle places ovulation around day 14. But cycles vary widely. Some run 24 days. Others run 35 or more.
If your cycle is longer than 28 days, you likely ovulated later. That pushes your actual conception date forward and makes your due date a bit later than the default formula would suggest. The reverse applies for shorter cycles.
Entering your true average cycle length gives a more accurate estimate. If your periods are irregular and you cannot pin down an average, an ultrasound scan may date the pregnancy more reliably.
What 40 weeks and 280 days mean for pregnancy due dates
A full-term pregnancy lasts about 40 weeks from the LMP, which equals 280 days. That is roughly 9 calendar months plus one week.
This 280-day figure is an average, not a deadline. Healthy babies arrive anywhere from 37 weeks to 42 weeks. Only about 4% to 5% of babies are born on their exact due date.
Think of your due date as the center of a window. Your baby could reasonably arrive two weeks before or two weeks after that date, and both scenarios are considered normal by most healthcare providers.
Calculate Your Due Date by Conception Date
If you tracked ovulation with test strips, basal body temperature, or timed intercourse, you may have a solid estimate of when conception occurred. Enter that date, and the calculator adds 266 days to find your due date.
This method removes the guesswork around cycle length. It starts the clock closer to the actual biological beginning of pregnancy.
Ovulation, fertilization, and the date of conception
Ovulation releases an egg that survives about 12 to 24 hours. Sperm can survive in the reproductive tract for up to 5 days. Fertilization typically happens within a day of ovulation.
Because of this overlap, pinpointing the exact date of conception is difficult even if you tracked your cycle closely. The conception date you enter is still an estimate in most cases.
That said, if you only had intercourse on one specific day near ovulation, you can feel more confident about your date. The calculator treats whatever date you enter as the assumed moment of fertilization and counts forward from there.
Calculate Your Due Date by IVF Transfer Date
In vitro fertilization (IVF) provides the most precise starting information of any method. The embryo's age and the exact transfer date are documented by your fertility clinic.
Enter the transfer date and select whether a Day 3 or Day 5 embryo was transferred. The calculator adjusts automatically.
Embryo transfer and how due dates differ for IVF
A Day 5 embryo (blastocyst) is two days more developed at the time of transfer than a Day 3 embryo. The due date calculation accounts for this difference:
- Day 3 embryo: Due date = transfer date + 263 days
- Day 5 embryo: Due date = transfer date + 261 days
Both formulas bring the total equivalent gestational period to 40 weeks from a theoretical LMP date. Your fertility clinic likely already gave you an estimated due date. This calculator should match that closely, though your provider's date always takes priority.
The IVF method eliminates uncertainty about ovulation and conception timing. It is the most precise path to estimating a due date.
Early Pregnancy Ultrasound Scan and Due Dates
An ultrasound performed in the first trimester is the most reliable way to accurately date the pregnancy. The technician measures the embryo (called crown-rump length) and compares it to established growth charts.
How an ultrasound can refine your estimated due date
Early ultrasound dating is most accurate between 7 and 13 weeks of gestational age. During this window, embryos grow at a very consistent rate, making size a strong indicator of age.
If the ultrasound-based gestational age differs from your LMP-based estimate by more than a certain margin (usually 5 to 7 days in the first trimester), your provider may adjust your due date to match the scan.
Later ultrasounds are less precise for dating because babies start growing at individual rates. A third-trimester scan can be off by as much as two to three weeks. That is why early pregnancy is the best time for dating.
If you have irregular periods, do not remember your last menstrual period, or got pregnant while on birth control, an early ultrasound is often the primary tool for establishing your due date.
When and Why Your Due Date Change May Happen
A due date change is not unusual. It does not mean something is wrong. It means your care team has better information than the original estimate was based on.
Prenatal ultrasound scan adjustments in early pregnancy
The most common reason for a due date change is a first-trimester ultrasound that shows a gestational age different from what your LMP predicted. This happens often when:
- Your cycle length was uncertain or irregular
- You did not recall the exact first day of your last period
- Ovulation occurred earlier or later than the assumed day 14
When the discrepancy between the LMP date and ultrasound date exceeds about 7 days, most providers will revise the due date to match the scan. A smaller difference usually means the original date stands.
Your due date may also shift if a follow-up ultrasound early in pregnancy tells a different story than the first. Once your provider sets a confirmed due date, it rarely changes again after the first trimester.
How Accurate Is a Pregnancy Due Date Calculator Estimate
A due date calculator gives you a useful target, not a precise prediction. Pregnancy usually lasts between 37 and 42 weeks, and the 40-week mark is simply the statistical midpoint.
What due dates can and cannot tell you
What a due date can do:
- Provide a planning date for prenatal care milestones
- Help your provider schedule screenings and tests at the right gestational ages
- Give you a timeframe to prepare for your baby's arrival
What a due date cannot do:
- Predict the exact day your baby will be born
- Account for individual biological variation
- Replace medical evaluation of your pregnancy
Studies consistently show that only about 4% to 5% of babies are born on their exact due date. About 80% arrive within two weeks of it, either side.
The calculator estimates a date. Your health care provider monitors the full picture, including prenatal appointments, ultrasound scans, and your individual health factors. Always discuss your due date and pregnancy timeline with your provider, especially if you have concerns about pregnancy timing or pregnancy complications.
This tool is a planning aid, not a substitute for prenatal care.