How accurate is your due date? Less than most people expect. Only about 4–5% of babies are born on their estimated due date, and most arrive in the weeks before or after. A full-term birth runs from 39 weeks 0 days to 40 weeks 6 days, and birth from 37 weeks is classed as early term, so treat your due date as the centre of a window rather than a deadline. Get your own estimate in seconds with the due date calculator.
The short version: a first-trimester ultrasound is usually the most reliable way to date a pregnancy, a last menstrual period (LMP) date works well when your cycles are regular, and a known conception or IVF transfer date is the most precise of all. Below, we explain how each method works, how far apart two dates can be before your clinician changes yours, and why even a perfectly calculated date is still a guess.
Quick Answer: Which Method Is Most Accurate?
| Method | How it works | Accuracy |
|---|---|---|
| LMP (Naegele’s rule) | First day of last period + 280 days | Good if cycles are regular and about 28 days |
| First-trimester ultrasound | Crown-rump length compared with expected size for age | Generally the most accurate, especially with irregular cycles |
| Conception or IVF transfer date | Counted directly from a known date | Very accurate when the date is certain |
How Due Dates Are Calculated
Method 1: Last Menstrual Period (LMP)
Naegele’s rule adds 280 days (40 weeks) to the first day of your last period. It assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, you probably ovulated later, so the true date can fall later than the calculation suggests; with a shorter cycle it can fall earlier. A common adjustment is to add or subtract the difference from 28 days. LMP dating also depends on remembering the date correctly, and recent hormonal contraception or breastfeeding can make periods unreliable. See how doctors calculate gestational age for the full process.
Method 2: Ultrasound Dating
An early scan measures the baby from head to bottom (the crown-rump length, or CRL) and compares it with the expected size for each week. Babies of the same age are very similar in size in early pregnancy, which is why a first-trimester scan is generally the most reliable way to date a pregnancy, typically to within about 5–7 days. It is especially helpful when your cycles are irregular or you are unsure of your last period. The later the scan, the less useful it is for dating, because babies grow at different rates as pregnancy goes on.
Method 3: Conception or IVF Transfer Date
If you know your conception date or had IVF, the due date is counted from it. For IVF, ACOG advises using the age of the embryo: a day-5 transfer gives a due date 261 days after transfer, and a day-3 transfer 263 days after. Try the conception calculator or the IVF due date calculator, and read due date vs. conception date to see why the numbers differ.
When Will Your Due Date Change?
Your clinician compares the LMP date with the early scan. If the two agree within a small margin, the LMP date stays. If the gap is larger, the ultrasound date replaces it. ACOG sets the margin by how far along you are at the scan, and the earlier the scan, the smaller the allowed difference:
| Gestational age at ultrasound | Measurement | Date is changed if the gap is more than |
|---|---|---|
| Up to 8 weeks 6 days | Crown-rump length | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | Crown-rump length | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | Head, abdomen and femur measurements | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | Head, abdomen and femur measurements | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | Head, abdomen and femur measurements | 14 days |
| 28 weeks 0 days and later | Head, abdomen and femur measurements | 21 days |
For example, if your LMP date and an 8-week scan differ by 3 days, you keep the LMP date. If they differ by 8 days, the scan date becomes your due date. Once a date is set by an early ultrasound it rarely changes again, even if the baby later measures big or small, because later measurements reflect growth, not age. You can compare growth using the fetal weight percentile calculator. For more detail, read when gestational age gets adjusted.
Why Babies Rarely Arrive on the Due Date
Even a perfectly calculated date cannot predict the day labor starts. In a well-known study of 125 healthy, naturally conceived pregnancies in which ovulation was measured precisely, the average pregnancy lasted 268 days from ovulation to birth (38 weeks 2 days), yet individual pregnancies varied by as much as 37 days. The researchers also linked the timing of implantation and early progesterone rise to how long a pregnancy lasted. In other words, normal pregnancy length varies from person to person for biological reasons, not because the date was miscalculated.
Understanding the Birth Window
| Term | Weeks |
|---|---|
| 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 |
| Post-term | 42 weeks 0 days and beyond |
These categories follow ACOG definitions. As your date approaches, read our labor guide and Braxton Hicks vs. real contractions.
Factors That Can Shift Timing
- First pregnancies often run slightly past the estimated date.
- A cycle length that differs from 28 days can make an LMP-based date too early or too late.
- Twins and other multiples often arrive earlier, commonly before 38 weeks.
- Medical conditions such as preeclampsia or gestational diabetes may lead to a planned earlier delivery.
- Natural variation in implantation timing and hormones, which no calculator can predict.
What to Do
- Calculate your date with the due date calculator and confirm it at your dating scan.
- Write down the first day of your last period now, and your cycle length, so your clinician has accurate numbers.
- Track your weeks with the weeks-to-months chart.
- Pack your bag by about 36 weeks using the hospital bag checklist.
Frequently Asked Questions
How accurate is an ultrasound due date?
A first-trimester ultrasound is generally the most accurate dating method, typically within about 5–7 days. Accuracy falls the later in pregnancy the scan is done.
Is LMP or ultrasound more accurate for my due date?
If your cycles are regular and you know your period date, the two usually agree closely. If they differ by more than the margin in the table above, your clinician will use the ultrasound date. With irregular cycles, the early ultrasound is usually the better guide.
Can my due date change?
Yes, mainly at the first or dating scan. After an early ultrasound sets your date, it rarely changes.
Why is my ultrasound due date different from my LMP due date?
Naegele’s rule assumes a 28-day cycle and ovulation on day 14. If you ovulated earlier or later, the LMP date will be off, and the scan measurement picks that up.
What percentage of babies are born on their due date?
Only about 4–5%. Most babies arrive within the two weeks either side.
Is 37 weeks too early?
Birth at 37 weeks is classed as early term, not preterm, though your clinician may advise on the best timing for you.
Skip the Math
Enter your last period, conception or transfer date in the due date calculator for your estimated due date and current week.
Sources
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date
- ACOG: Definition of Term Pregnancy
- Jukic et al., Human Reproduction (2013): Length of human pregnancy and contributors to its natural variation
- StatPearls: Sonography Assessment of Gestational Age
- AIMS: How accurate is my due date?
- PregCalc medical review process
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN or midwife about your own pregnancy.
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