Enter the first day of your last menstrual period and the calculator instantly shows your estimated due date, current gestational age, trimester, and key upcoming milestones. It also works from conception date, a known due date, ultrasound measurements, or an IVF transfer date. Use the date lookup at the bottom to check how far along you'll be on any future date.
- A standard pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period, using Naegele's rule (ACOG, 2017).
- Only about 5% of babies are born on their exact due date - most deliver within a 2-week window around the estimate.
- First-trimester ultrasound (crown-rump length) is the most accurate dating method, with a margin of error of just 5-7 days.
- IVF due dates are the most precise: transfer date, embryo age, and fertilization timing are all exactly known.
- Use the date lookup at the bottom to instantly check how far along you'll be on any future date - useful for planning events, travel, or appointments.
How to use this pregnancy due date calculator
Select your calculation method from the five tabs, enter the relevant date, and results appear instantly. The Last Period method is the default and requires only one date. For ultrasound, enter both the scan date and the gestational age shown on your report. For IVF, select whether you transferred a Day 3, Day 5, or Day 6 embryo.
All five methods produce the same set of outputs: estimated due date, current gestational age in weeks and days, trimester, days remaining, a fetal size comparison, upcoming key milestones, and a complete key-dates timeline.
Below the key dates grid, there is also a date lookup: enter any date to instantly see your gestational age, trimester, and fetal size on that day. This is useful for planning - for example, checking how far along you'll be at a wedding, a work trip, or a holiday.
How to calculate your due date
The standard clinical method is Naegele's rule, first published in 1812 and still used worldwide today: add 280 days (40 weeks) to the first day of your last menstrual period.
If your cycle differs from 28 days, the calculator adjusts automatically - a 30-day cycle pushes the due date 2 days later, for instance. This simple formula underlies every method in this calculator.
The formula counts pregnancy in gestational age, which starts two weeks before conception. That's why a newborn delivered at "40 weeks" has technically only been developing for about 38 weeks.
Gestational age is the universal clinical standard because most people know when their last period started, while exact conception dates are often unknown.
Gestational age explained to a beginner
Think of gestational age like a school year that starts on September 1st, even though class doesn't actually begin until September 14th. The "year" starts before any real learning happens.
Pregnancy works the same way: doctors start counting from your last period (September 1st), even though conception (the first day of actual "class") happens about two weeks later.
So when your doctor says you're 8 weeks pregnant, the baby has only been developing for about 6 weeks - but everyone uses the 8-week number because it's based on your period date, which is easy to track.
Which calculation method should you use?
The right method depends on what information you have. First-trimester ultrasound is the gold standard, accurate to within 5-7 days (ACOG Committee Opinion 700, 2017).
LMP-based dating (last period method) is the most widely used when ultrasound isn't available, but it assumes you know your period date and assumes regular ovulation on day 14.
| Method | When to use it | Typical accuracy | Key limitation |
|---|---|---|---|
| Last Period (LMP) | You know your last period date | +/- 2 weeks | Assumes regular 28-day cycle and ovulation on day 14 |
| Conception Date | You know when you conceived | +/- 1-2 weeks | Exact conception date is rarely known from natural intercourse |
| Known Due Date | Your provider already gave you a date | Varies | You need the date from a clinical source, not a guess |
| Ultrasound | You have a scan report with gestational age | +/- 5-7 days (1st trimester) | Accuracy decreases after week 20; third-trimester scans are +/- 3 weeks |
| IVF Transfer | You had an embryo transfer (Day 3, 5, or 6) | +/- 1-3 days | Accounts only for transfer date; implantation still varies slightly |
In practice, most care providers use LMP (last menstrual period) for initial dating and then confirm or adjust with a first-trimester ultrasound. If the two methods differ by more than 7 days, the ultrasound measurement typically takes precedence.
When I've worked through due date questions with people, the most common mistake is using an LMP date from memory rather than checking a calendar or app. Even being off by one week on the LMP date moves the due date by a full week - which can affect decisions about induction timing later in pregnancy.
What is gestational age exactly?
Gestational age is the age of a pregnancy measured in weeks and days from the first day of the last menstrual period. It's the universal language of obstetrics - every screening test, viability threshold, and developmental milestone is expressed in gestational weeks.
A pregnancy that reaches 40 weeks gestational age is considered full term, though 37-41 weeks is the normal delivery range (WHO, 2015).
Gestational age differs from fetal age (also called fertilization age or embryonic age) by approximately two weeks. Fetal age counts from conception; gestational age counts from LMP.
When a scan report says your baby is "22 weeks 3 days," that's gestational age. When a fetal development book says "at 20 weeks the baby can hear," that may be measuring fetal age - so be aware of which measure a source is using.
| Trimester | Weeks | Key developments | Key screenings |
|---|---|---|---|
| First | 1-13 | Neural tube, heart, limb buds form; heartbeat detectable ~week 6 | Nuchal translucency scan (10-13 wks); NIPT/cell-free DNA |
| Second | 14-27 | Baby grows from lemon (week 14) to head of cauliflower (week 27); movement felt ~week 18-20 | Anatomy scan (18-22 wks); glucose screening (24-28 wks) |
| Third | 28-40+ | Lungs mature; baby reaches watermelon size by week 40; position usually set by week 32 | Group B strep test (35-37 wks); weekly monitoring from week 36 |
How big is your baby? Fetal size by trimester
Fetal size comparisons to fruit are a widely used way to visualize growth - and they're clinically grounded in crown-rump length (CRL) measurements taken by ultrasound.
At week 8, the embryo is about 16 mm long, roughly the size of a raspberry. By week 20 it's a 161-mm banana. At term (week 40), the average newborn is about 50 cm and 3.5 kg (19.7 in and 7.7 lbs) - the size of a watermelon.
The size displayed in the calculator is based on median CRL and biparietal diameter data from standard obstetric growth charts.
Individual babies vary significantly - a baby measuring slightly smaller or larger than the median is completely normal. Your sonographer will flag any measurements outside the normal range.
| Week | Size comparison | Crown-rump length | Approximate weight |
|---|---|---|---|
| 8 | Raspberry | ~16 mm | ~1 g |
| 10 | Kumquat | ~31 mm | ~4 g |
| 12 | Lime | ~54 mm | ~14 g |
| 16 | Avocado | ~116 mm | ~100 g |
| 20 | Banana | ~161 mm | ~300 g |
| 24 | Ear of corn | ~300 mm | ~600 g |
| 28 | Eggplant | ~370 mm | ~1.0 kg |
| 32 | Jicama | ~430 mm | ~1.7 kg |
| 36 | Romaine head | ~470 mm | ~2.6 kg |
| 40 | Watermelon | ~510 mm | ~3.5 kg |
Use our centimeters to inches converter and our kilos to pounds converter for imperial units.
How accurate is your due date?
Due dates are estimates, not predictions. Only about 5% of babies arrive on the exact EDD (Mongelli et al., 1996). Most full-term deliveries happen between weeks 37 and 41.
Accuracy depends heavily on the method used: first-trimester ultrasound is the most reliable, while LMP-based dating assumes ovulation on exactly day 14 of a perfect 28-day cycle - which doesn't describe most people's cycles.
ACOG recommends adjusting the LMP-based EDD if ultrasound dating differs by more than 7 days in the first trimester, 10 days in the second, or 14 days in the third. So if your LMP puts you at 10 weeks but your ultrasound shows 11 weeks 2 days, your provider will update your due date to match the scan.
After 20 weeks, ultrasound accuracy drops significantly - a third-trimester scan can be off by up to 3 weeks.
One thing I've seen often: people use the calculator to track their due date, see it's a Thursday, and mentally commit to that date. The healthier framing is to think of a 4-week window - "sometime between [date - 2 weeks] and [date + 2 weeks]" - and plan accordingly.
Having baby gear, support, and leave arranged by week 37 is far more useful than fixating on a single day.
FAQs about due date and gestational age
How is the due date calculated from the last period?
Using Naegele's rule, the estimated due date is the first day of your last menstrual period plus 280 days (40 weeks). This formula assumes a 28-day cycle. For longer or shorter cycles, the calculator adjusts by the difference from 28 days - so a 30-day cycle pushes the EDD 2 days later, and a 25-day cycle moves it 3 days earlier.
What is gestational age, and how is it different from fetal age?
Gestational age counts from the first day of your last period; fetal age (embryonic age) counts from conception. They differ by about 2 weeks. All clinical milestones - viability at week 24, full term at week 37, post-term at week 42 - use gestational age. When a book or app says "your baby at week X," check whether it's using gestational age or fetal age to avoid confusion.
How accurate is an ultrasound due date compared with LMP dating?
A first-trimester ultrasound measuring crown-rump length is accurate to within 5-7 days and is considered the gold standard (Kalish et al., 2004). LMP dating can be off by up to 2 weeks, especially with irregular cycles or late ovulation. ACOG recommends adjusting the due date to match the ultrasound if the two methods differ by more than 7 days at under 14 weeks.
How does an IVF transfer date change the calculation?
For IVF pregnancies, gestational age on transfer day equals 2 weeks plus the embryo's age (14 + 3 = 17 days for Day 3; 14 + 5 = 19 days for Day 5; 14 + 6 = 20 days for Day 6). The equivalent LMP is set by subtracting that gestational age from the transfer date. IVF due dates are the most accurate of all methods because fertilization timing is precisely documented.
What does "full term" mean, and when is it safe to deliver?
Full term begins at 37 completed weeks of gestational age. Deliveries from 37 to 38+6 weeks are called "early term," 39-40+6 weeks are "full term," and 41-41+6 weeks are "late term." Post-term is 42 weeks or more, when induction is typically offered. The optimal delivery window for most uncomplicated pregnancies is 39-40 weeks, when neonatal outcomes are best (ACOG, 2013).
Quiz: how well do you know pregnancy due dates?
1. How many days does a standard pregnancy last from the last menstrual period (LMP)?
2. If you transfer a Day-5 blastocyst via IVF, what gestational age does your doctor assign on transfer day?
3. Which due date method is considered the most accurate?
4. What percentage of babies are actually born on their estimated due date?
5. When does the second trimester begin?