PregnancyDue Date Calculator — When Is Your Baby Due?
Enter the first day of your last menstrual period and get your estimated due date, current pregnancy week, trimester, and a countdown to delivery — instantly and free.
Pregnancy Due Date Calculator
How Is a Due Date Calculated?
The standard method used by doctors worldwide is Naegele's rule, published in 1812 and still the default today. Here’s how it works. You take the first day of your last menstrual period (LMP) and add 280 days (or 40 weeks). That gives you your estimated due date (EDD).
Naegele’s rule assumes a 28-day menstrual cycle. It assumes ovulation happens on day 14. If your cycle is longer, ovulation probably happened later. That usually shifts the due date forward. If your cycle is shorter, ovulation was earlier, pulling the due date back. The calculator above does this automatically.
The formula looks simple, but here's the thing, pregnancy isn't actually 40 weeks from conception. It's 40 weeks from the first day of your last period. That’s usually about 2 weeks before you conceived. Gestational age includes those two weeks before conception. That’s why you’re technically “two weeks pregnant” at the time of conception.
So, how accurate are due dates?
It doesn't feel very intuitive, and that's normal. Only about 4% of babies arrive on their exact estimated due date. The EDD is better understood as the center of a window rather than a precise target.
Most babies are born between 38 and 42 weeks of gestation. A study published in BMJ found that the median duration of pregnancy for first-time mothers was 283 days from LMP — three days past the standard 280-day estimate. For second pregnancies and beyond, the median was closer to 280 days.
Several factors affect accuracy:
- Irregular cycles. If your periods don't come like clockwork, an LMP-based calculation can be off by a week or more.
- Late ovulation. Even with a regular cycle, ovulation doesn’t always happen on day 14. A few days' variance shifts the true conception date.
- Recall error. If you aren’t sure about the exact start date of your last period, that uncertainty carries through to your due date.
- First vs. subsequent pregnancies. First babies tend to arrive a few days later than the EDD on average.
An early ultrasound (before 13 weeks) can usually narrow the due date window to within 5–7 days. That’s why most providers confirm or adjust your due date at the first-trimester scan. They compare the ultrasound measurement with the LMP calculation. When the two disagree by more than seven days, the ultrasound date is typically used.
Last Menstrual Period vs. Conception Date
Due date calculators almost always ask for the first day of your last menstrual period — not the date of conception. The reason is practical: most women know when their period started but don't know exactly when they conceived.
Conception usually happens around ovulation. Ovulation usually occurs roughly 14 days before the start of your next period. For a 28-day cycle, ovulation is around day 14. For a 32-day cycle, ovulation is usually closer to day 18. Sperm can survive inside the body for up to five days. So yeah, it’s rare to pinpoint the exact moment of fertilization.
If you do know the conception date — perhaps you used ovulation tracking or IVF — you can calculate the due date by adding 266 days (38 weeks) from that date. This bypasses the uncertainty around cycle length and ovulation timing. Some fertility clinics date pregnancies this way and then convert to gestational age by adding the standard two weeks.
Both methods aim for the same delivery window. The difference is which starting point you use. Track your pregnancy week by week using our pregnancy tracker once you have an estimated due date.
What If I Don't Know My LMP?
This is common — and it doesn't mean your due date will be a guess. Your provider has other tools.
The most reliable alternative is a first-trimester ultrasound. Between 8 and 13 weeks, the ultrasound tech measures the baby’s crown-rump length (CRL). In early pregnancy, embryos grow at nearly the same rate. That’s why a CRL measurement usually gives a gestational age within 5–7 days.
If you come in later, during the second trimester, the ultrasound can still estimate gestational age. The tech does this by measuring the baby’s head circumference, femur length, and abdominal circumference. These measurements become less precise as pregnancy progresses because individual growth variations widen. A second-trimester scan is generally accurate to within 10–14 days.
Providers might also use other clues. These can include the date of a positive pregnancy test. They can include when you first heard the heartbeat. They can include when you first felt the baby move (quickening). None of these methods is as reliable as an early ultrasound. But in most cases, they do help narrow the range.
Bottom line, if you don’t remember when your last period started, schedule an early ultrasound. The sooner it’s done, the more accurate the dating is.
Due Date by Ultrasound
Ultrasound dating is considered the gold standard when LMP-based dating is uncertain. In the first trimester, the embryo grows predictably. A single measurement can pinpoint gestational age with high confidence.
Here's how accuracy changes with timing:
- Before 9 weeks: accurate to ±5 days
- 9 to 13 weeks: accurate to ±7 days
- 14 to 20 weeks: accurate to ±10 days
- After 20 weeks: accurate to ±14–21 days
The American College of Obstetricians and Gynecologists (ACOG) recommends that if the ultrasound-based EDD and LMP-based EDD differ by more than 7 days in the first trimester (or more than 10 days in the second trimester), the ultrasound date should be used. Once your due date is confirmed, it generally shouldn't be changed again.
This matters because gestational age guides clinical decisions. It affects when to screen for genetic conditions. It affects when labor can safely be induced. It affects when a pregnancy is considered post-term. An inaccurate due date can lead to unnecessary interventions or missed ones.
What Is a Full-Term Pregnancy?
ACOG refined the definition of "full term" in 2013 to give clearer guidance on delivery timing. The categories are:
- 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
Babies born at 39–40 weeks have the best outcomes on average. Their lungs, brain, and liver have had the most time to mature. Early-term babies (37–38 weeks) are generally healthy. But babies born at 39–40 weeks have slightly higher rates of breathing problems, feeding difficulties, and NICU admission than full-term babies.
Post-term pregnancies (42+ weeks) come with increased risks. These risks include placental aging. Most providers recommend induction or increased monitoring by 41 weeks. The ARRIVE trial (2018) found that induction at 39 weeks in low-risk pregnancies slightly reduced certain complications.
But none of this means you should try to time your delivery. The categories help your provider make informed recommendations. Your baby's movement patterns remain one of the most important things to monitor as you approach your due date.
Week-by-Week Pregnancy Tracking
Once you have a due date, counting your pregnancy week becomes straightforward. Gestational age starts on the first day of your last period, week 1, day 1. Here's a brief overview of what happens during each trimester:
First Trimester (Weeks 1–12)
The embryo implants in the uterine wall around week 4. By week 6, the heart begins to beat. By week 8, all major organs are forming. In most cases, the risk drops a lot after a heartbeat is confirmed. Most women get nausea, fatigue, and breast tenderness during this period.
Second Trimester (Weeks 13–26)
Often called the "easiest" trimester. Nausea usually fades, energy returns, and the baby bump becomes visible. Around week 18–22, the anatomy scan checks the baby's development in detail. Most women feel the first kicks (quickening) between weeks 16 and 25. This is also when the baby's sex can typically be identified.
Third Trimester (Weeks 27–40)
The baby gains weight rapidly — roughly half a pound per week. The lungs mature. The baby usually settles into a head-down position for birth by week 36. Common symptoms include back pain, frequent urination, Braxton Hicks contractions, and trouble sleeping. By week 37, the baby is considered early term.
Tracking pregnancy week by week helps you see what’s usually normal at that stage. It also helps you spot when it’s time to bring something up with your provider. The Pregnancy App pregnancy tracker gives you personalized weekly updates based on your due date.
TL;DR
- A due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period — adjusted for cycle length.
- Only 4% of babies arrive on their exact due date. Most are born between 38 and 42 weeks.
- A first-trimester ultrasound is the most accurate dating method (±5–7 days).
- Full term is 39–40 weeks. Early term is 37–38. Post-term is 42+.
- If you don't know your LMP, an early ultrasound can give you a reliable due date.
Limitations & Safety
This due date calculator is an educational tool, not a medical device. It gives an estimate based on Naegele's rule and the information you enter. It doesn’t account for individual biological variation. It doesn’t account for irregular cycles beyond the adjustment range. It doesn’t account for medical conditions that can affect gestational length.
The calculator can’t replace a clinical assessment. Your healthcare provider uses a combination of LMP dates, ultrasound measurements, physical examination, and your medical history to determine the most accurate estimated due date. If the calculator’s result is very different from the date your provider gives you, use your provider’s date.
Don’t use this tool to decide on inducing labor, scheduling a cesarean, or timing any medical procedure. These decisions need to involve your obstetrician or midwife. If you have bleeding, severe pain, reduced fetal movement, or fluid leaking before your due date, contact your provider or go to the hospital right away. Do it no matter how many weeks the calculator says you have left.
Frequently Asked Questions
How is a pregnancy due date calculated?
The most common method is Naegele’s rule. You add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle is longer or shorter than 28 days, the due date calculation is adjusted by adding or subtracting the difference. For example, a 30-day cycle adds 2 extra days to the estimated due date.
How accurate is a due date calculator?
Only about 4% of babies are born on their exact due date. The estimated due date is the midpoint of a normal delivery window. Most babies arrive between 38 and 42 weeks. A first-trimester ultrasound is usually accurate within 5–7 days. LMP-based calculations can be off by up to two weeks.
What if I don't know my last menstrual period?
If you don't know the date of your last period, your provider can estimate your due date with an ultrasound. A first-trimester ultrasound (before 13 weeks) measures the baby's crown-rump length. It's usually the most accurate way to date a pregnancy when your LMP is unknown or unreliable.
Can my due date change?
Yes. Your provider may adjust your due date after an early ultrasound if the baby's measurements differ from the LMP-based estimate by more than 7 days. Once a due date is confirmed by a first-trimester ultrasound, it typically isn't changed again.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period. Gestational age is usually about two weeks longer than fetal age. Fetal age (also called embryonic age) is counted from the estimated date of conception. Medical providers use gestational age as the standard measurement.
Is a due date based on conception actually more accurate?
If you know the exact date of conception, your due date can be slightly more accurate. However, most women do not know the precise conception date. LMP-based calculations that are adjusted for cycle length usually work well for most pregnancies.
What counts as full-term pregnancy?
A pregnancy is considered early term at 37–38 weeks. Babies born at 39–40 weeks generally have the best health outcomes.
Does cycle length affect my due date?
Yes. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, you likely ovulated later, which pushes the due date forward. If your cycle is shorter, ovulation probably happened earlier, so the due date shifts earlier too. A due date calculator that accounts for cycle length gives a more accurate estimate.