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, attributed to German obstetrician Franz Naegele in the early 1800s. It's still the most widely used formula today, and an average pregnancy is calculated as lasting 280 days (40 weeks) from the first day of the last menstrual period.
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 count from the last period? Because the exact day of conception is rarely known, clinicians anchor the count to the first day of the LMP. Since ovulation in a textbook 28-day cycle happens about 2 weeks after the LMP, the 40-week total includes roughly 2 weeks before you were actually pregnant — so the baby's true fetal age at a 40-week birth is about 38 weeks.
Naegele's Rule assumes a regular 28-day cycle with ovulation on day 14. If that doesn't describe you, see "Factors That Affect Your Due Date" below.
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 a first-trimester ultrasound (before 13 weeks 6 days) is the most accurate method to establish or confirm gestational age, with an accuracy of about ±5 to 7 days, because it directly measures the embryo's crown-rump length rather than assuming a textbook cycle.
Whether your provider keeps the LMP date or switches to the scan depends on how far apart the two estimates are. Dating guidelines change the LMP date to match the ultrasound only when the gap exceeds a threshold that widens as pregnancy advances — because ultrasound itself becomes less precise later on:
| Gestational age at ultrasound | Redate if LMP and scan differ by more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days – 13 weeks 6 days | 7 days |
| 14 weeks 0 days – 15 weeks 6 days | 7 days |
| 16 weeks 0 days – 21 weeks 6 days | 10 days |
| 22 weeks 0 days – 27 weeks 6 days | 14 days |
<small>Thresholds per peer-reviewed pregnancy-dating guidance summarized by NIH StatPearls. The first-trimester crown-rump-length window closes at 13 weeks 6 days, so the 9–13w6d and 14–15w6d brackets share the same 7-day threshold but are listed separately.</small>
The same StatPearls review notes that a third-trimester scan (beyond 28 weeks 0 days) is the least reliable dating method, accurate only to about ±21–30 days, so it is generally not used to redate an established due date.
This is why an early scan matters so much: a measurement at 8 weeks can revise your date over a 5-day disagreement, while a third-trimester scan rarely changes an established due date at all.
Understanding Your Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones:
| Trimester | Weeks | Key Developments |
|---|---|---|
| First | 1–13 | Heart begins beating (week 6), major organs form, morning sickness peaks |
| Second | 14–27 | Baby kicks felt (week 18–22), gender can be identified, rapid growth |
| Third | 28–40 | Lungs mature, baby gains weight, brain development accelerates |
ACOG defines the trimesters as the first running through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third beginning at 28 weeks 0 days. 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, because the embryo's exact age is known. With a day-3 transfer the EDD is 263 days from the transfer date; with a day-5 blastocyst transfer you add 261 days — the two-day difference reflects the extra growth a blastocyst has already done in the lab. Because this dating is so precise, an IVF due date is generally not overridden by a later ultrasound.
Multiple Pregnancies
Multiples are typically delivered earlier than a single baby, and the target timing depends on the type of pregnancy rather than a single fixed week. Under ACOG's guidance on twin, triplet, and higher-order pregnancies, twins that share neither a placenta nor a sac (dichorionic-diamniotic) are generally managed toward about 38 weeks; twins that share a placenta but have separate sacs (monochorionic-diamniotic) toward roughly 36–37 weeks; and twins sharing a single sac (monochorionic-monoamniotic) considerably earlier, around 32–34 weeks. Triplet and higher-order pregnancies are also delivered preterm, often in the low-to-mid 30s of weeks. Because these ranges vary so much, your OB will monitor you closely and set an individualized 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: very few babies arrive on their exact estimated due date. Most births occur within a roughly two-week window on either side of the EDD, which is why a single calendar date can be misleading.
That's why obstetricians don't treat "40 weeks" as a single deadline. A work group convened by ACOG and the Eunice Kennedy Shriver National Institute of Child Health and Human Development standardized the labels ACOG now uses:
| Label | Gestational age |
|---|---|
| Early term | 37 weeks 0 days – 38 weeks 6 days |
| Full term | 39 weeks 0 days – 40 weeks 6 days |
| Late term | 41 weeks 0 days – 41 weeks 6 days |
| Postterm (overdue) | 42 weeks 0 days and beyond |
So your due date is best understood as the center of a delivery window, not a fixed appointment.
Why Your Due Date Matters Beyond the Countdown
Even though almost no one delivers on the exact day, an accurate due date does real work. It sets the timing for routine prenatal milestones — first-trimester screening, the anatomy ultrasound around 18–22 weeks, gestational-diabetes screening, and Group B strep testing all key off your gestational age, so getting the date right early keeps every later test in its proper window.
It also defines what counts as on-time or overdue. Because the official "full term" window is 39 weeks 0 days through 40 weeks 6 days, providers generally avoid scheduling non-medically-indicated deliveries before 39 weeks and begin discussing monitoring or induction once a pregnancy reaches the late-term and postterm range. A date that's off by a week can shift those conversations — one more reason an early scan is so valuable.
Finally, it's the reference point for practical planning: parental leave, childcare, a hospital bag, and a birth plan. Anchoring those to a 37-to-42-week range rather than a single date tends to reduce last-minute stress.
A note on health information: This article is general educational information about how pregnancy due dates are estimated — it is not medical advice and is not a substitute for care from a qualified healthcare provider. Due dates, dating methods, and delivery timing should always be confirmed and managed by your own doctor or midwife, who can account for your individual health history.
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.
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 provider will use an early ultrasound to establish your due date. This is common and works well — a first-trimester scan is the most accurate dating method regardless of whether you know your LMP.
Calculate Your Due Date Now
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