Finding out you're pregnant is one of the most overwhelming moments of your life — and one of the first questions that pops up is simple: when is my due date?
Good news: you can figure it out in under a minute. This calculator uses Naegele's rule — the standard medical formula that adds 280 days (40 weeks) to the first day of your last menstrual period. It's the same method your OB or midwife uses at your first appointment.
How Accurate Is This Calculator?
It's a solid estimate — but an estimate is all it is. Only about 4–5% of babies are born on their exact due date. Most arrive anywhere between 37 and 42 weeks, and that whole window is considered full-term.
The Naegele formula assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, your actual conception date may fall on a different day, which can shift your true due date.
If you have irregular cycles, don't know your LMP, or conceive while using hormonal contraception, your OB may use an early ultrasound to date your pregnancy instead. First-trimester ultrasounds (before 13 weeks) are quite accurate for pinpointing gestational age.
📅 Note on dating: The "first day of your last period" means the first day you had bleeding — not when it ended, not when you think ovulation happened, not when you had sex. This is the standard medical convention and the only date that reliably goes into the formula.
When Did Conception Actually Happen?
The due date formula says 280 days from LMP — but conception itself occurs around day 14 of a typical cycle. That means your baby is actually only about 266 days old at birth (38 weeks from ovulation, not 40 weeks from LMP). The extra two weeks are pre-conception — the time between your period starting and ovulation happening.
Because ovulation timing varies significantly between individuals, if you know your actual ovulation date (from temping, ovulation strips, or fertility tracking), your provider may use that to calculate your due date instead. If you've been tracking ovulation, mention it at your first prenatal visit.
What Happens at Each Trimester
Your due date gives you a finish line — but knowing what's actually happening inside your body along the way makes the journey feel less abstract. Here's a quick map of the three trimesters:
Trimester 1
The Foundation
Weeks 1–13
All of baby's major organs form, the heartbeat starts around week 6, and placental development happens here. For you: nausea, fatigue, breast tenderness, and emotional volatility — read our week-by-week first trimester symptoms guide for what to expect. The highest miscarriage risk window is early — but once you hear that heartbeat, the risk drops dramatically.
Trimester 2
The Golden Period
Weeks 14–26
Most people feel significantly better during the second trimester — energy returns, nausea eases, and you start to "pop." You'll feel first movements (quickening) between 16–22 weeks. Baby's hearing develops, and gender can be seen on ultrasound. Many parents find out the sex here.
Trimester 3
The Final Push
Weeks 27–40+
Baby gains most of its birth weight here — about half a pound per week in the final weeks. You'll feel bigger, more uncomfortable, and more tired. Braxton Hicks contractions increase. By week 36, baby is typically head-down and ready for delivery. Many first-time parents go past their due date; it's normal and usually not a problem.
What If My Due Date Changes?
It happens more often than you'd think. If an early ultrasound shows a different date than your LMP calculation, your provider will discuss updating your due date. The earlier the ultrasound, the more reliable the dating. After 20 weeks, the margins of error grow larger, so changes become less common.
Here's what this means practically:
- If the ultrasound is within 5 days of your LMP date: Your original due date likely stays. First-trimester scans within this window are common and don't typically trigger a change.
- If the difference is larger: Your provider may shift your due date. This is not a sign anything is wrong — it just means the math is being recalibrated with better information.
Don't be surprised or alarmed if your due date shifts by a few days or even a week. It's standard medical practice and reflects better data, not a problem.
What a Due Date Doesn't Tell You
A due date gives you a target — but it doesn't predict:
- When you'll go into labor (spontaneous labor is still unpredictable)
- Whether you'll need induction or a planned cesarean
- How long your labor will last
- Whether baby will come before 37 weeks (preterm) or after 42 weeks (post-term)
Think of your due date as an estimate, not a deadline. It's a helpful reference point for monitoring growth, scheduling tests, and knowing when to call your provider if something feels off — but it's not a guarantee of anything.
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When to Call Your Provider
Contact your OB or midwife if:
- You experience heavy bleeding (soaking a pad), severe abdominal pain, or sudden gush of fluid
- You have fever above 100.4°F (38°C)
- You haven't had any prenatal care by 10–12 weeks — it's time to get started
- You're unsure about any medications, symptoms, or health concerns during pregnancy
If you're looking for a guide that walks you through each week with real, practical advice — not just a list of what's happening to your body — CradleWise is built exactly for that. Explore subscription plans or start with our symptoms guide to understand what's actually normal to feel.
Get Erin's Week-by-Week Pregnancy Guide
Know what's happening, what's normal, and when to call your doctor — for every week of your pregnancy.
Frequently Asked Questions
How accurate is a due date based on my last period?
A due date calculated from the first day of your last menstrual period (Naegele's rule) is an estimate — not a promise. Only about 4–5% of babies are born on their exact due date. Most arrive somewhere between 37 and 42 weeks. If you have irregular cycles, a later ultrasound may give a more accurate dating.
What if I don't know the first day of my last period?
If you don't know your LMP or have very irregular cycles, your OB or midwife can estimate your due date using an early ultrasound — typically done in the first trimester (before 13 weeks). Crown-rump length measurements at that stage are very accurate for dating. Don't panic if your initial due date shifts — it's common and normal.
Does conception happen on day 14 of my cycle?
The traditional Naegele formula assumes conception occurs around day 14 — but this is an average based on a 28-day cycle with ovulation on day 14. In reality, ovulation timing varies significantly by person and by cycle. Women with longer cycles ovulate later, women with shorter cycles ovulate earlier. If you know your actual ovulation date, your provider may adjust your due date accordingly.
Can my due date change after an ultrasound?
Yes — and it's more common than most people realize. An ultrasound in the first trimester (before 12 weeks) gives a very accurate picture of gestational age based on the embryo's size. If there's a significant difference between the ultrasound dating and the LMP-based date, your provider may adjust your due date. Second-trimester ultrasounds are less reliable for dating, so changes after 20 weeks are less common.
What happens at each trimester?
The first trimester (weeks 1–13) covers early development: organ formation, the heartbeat starting, and the placenta taking over. The second trimester (weeks 14–26) is often called the "golden period" — more energy, visible movement, and preparation. The third trimester (weeks 27–40+) is when your baby gains most of their weight and your body prepares for labor.
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