Finding out you're expecting is one of life's most exciting moments — and one of the first questions every parent-to-be asks is: "When is my baby due?"
Our free pregnancy due date calculator gives you an instant estimate based on your last menstrual period (LMP), conception date, or IVF transfer date. But how exactly does the math work, and how accurate is it?
This guide breaks it all down.
How Due Dates Are Calculated: Naegele's Rule
The standard method for estimating a due date is Naegele's Rule, developed by German obstetrician Franz Naegele in the early 1800s. It's still the most widely used formula today.
The formula is simple:
That's it. The result is your estimated due date (EDD).
For example, if your LMP was January 1, 2026, your estimated due date would be October 8, 2026.
Why 280 days? A typical pregnancy lasts about 40 weeks from the first day of the LMP. This includes roughly 2 weeks before ovulation even occurs, so the actual fetal age is about 38 weeks at birth.
Try it now → Use our pregnancy due date calculator to get your EDD instantly.
LMP vs. Ultrasound: Which Is More Accurate?
Not all due date methods are created equal. Here's how the two most common methods compare:
| Method | How It Works | Best Used When | Accuracy |
|---|---|---|---|
| LMP (Naegele's Rule) | Adds 280 days to last period date | Regular 28-day cycles | ± 2 weeks |
| First-Trimester Ultrasound | Measures fetal crown-rump length | 8–13 weeks gestation | ± 5–7 days |
| Second-Trimester Ultrasound | Measures head, abdomen, femur | 14–20 weeks gestation | ± 1–2 weeks |
Key takeaway: If you have regular periods, the LMP method is a solid starting point. But an early ultrasound (before 13 weeks) is considered the gold standard for accuracy because it directly measures the baby's size.
Your doctor may adjust your due date if the ultrasound measurement differs from the LMP estimate by more than 7 days.
Understanding Your Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones:
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1–12 | Heart begins beating (week 6), major organs form, morning sickness peaks |
| Second | 13–26 | Baby kicks felt (week 18–22), gender can be identified, rapid growth |
| Third | 27–40 | Lungs mature, baby gains weight, brain development accelerates |
Knowing which trimester you're in helps you prepare for upcoming appointments, screenings, and physical changes.
Pro tip: Use our pregnancy due date calculator — it automatically shows your current trimester and weeks along.
Factors That Affect Your Due Date
While Naegele's Rule works well for most pregnancies, several factors can shift your actual delivery date:
Irregular Cycles
If your menstrual cycle is longer or shorter than 28 days, your ovulation date shifts too. A 35-day cycle means ovulation likely occurred around day 21, not day 14. This can push your true due date later than the LMP estimate suggests.
IVF and Assisted Conception
For IVF pregnancies, the due date is calculated from the embryo transfer date rather than the LMP:
Multiple Pregnancies
Twins typically arrive around 36–37 weeks, while triplets often arrive around 32–34 weeks. Your OB will monitor you more closely and may set an earlier target delivery date.
First-Time vs. Subsequent Pregnancies
Statistically, first-time mothers tend to deliver slightly later than their due date, while subsequent pregnancies may arrive a few days earlier.
How Accurate Is a Due Date, Really?
Here's the truth: only about 4–5% of babies are born on their exact due date. Most births occur within a two-week window around the EDD.
A delivery is considered full term if it occurs between 39 and 40 weeks. Babies born between 37 and 38 weeks are "early term," while those born after 41 weeks are "late term."
Your due date is best understood as the center of a delivery window, not a precise appointment.
FAQ
How do I calculate my pregnancy due date from my last period?
Add 280 days (40 weeks) to the first day of your last menstrual period. This is called Naegele's Rule and is the standard method used by doctors worldwide. Our due date calculator does this instantly.
Can my due date change during pregnancy?
Yes — if a first-trimester ultrasound shows a measurement that differs from the LMP-based estimate by more than 7 days, your healthcare provider may adjust your due date to match the ultrasound.
How far along am I right now?
Your gestational age is measured from the first day of your last period to today. Enter your LMP in our due date calculator to see your exact weeks and days.
Is the due date from an ultrasound more accurate than LMP?
Generally, yes — especially if the ultrasound is performed in the first trimester (before 13 weeks). It measures the baby directly, while the LMP method assumes a standard 28-day cycle.
What if I don't know the date of my last period?
If you can't remember your LMP or have highly irregular cycles, your doctor will use an early ultrasound to establish your due date. This is common and perfectly reliable.
Calculate Your Due Date Now
Ready to find out when your little one might arrive? Our free, no-signup pregnancy due date calculator gives you:
It's private, fast, and works on any device. **Open the Calculator →