Educational content, not medical advice. Clinical review of this page is in progress.
Providers typically start with your last menstrual period, then may refine that estimate using ultrasound measurements — especially in the first trimester.
LMP-Based Dating
The default starting point, assuming a standard 28-day cycle with ovulation on day 14.
Ultrasound Dating
Crown-rump length (CRL), measured in the first trimester, is considered the most accurate way to date a pregnancy — especially useful when LMP is uncertain or cycles are irregular.
When Providers Adjust the Due Date
If ultrasound dating differs from LMP dating by more than about 5–7 days in the first trimester (or more in later trimesters), providers generally adopt the ultrasound-based due date.
Step by Step: How Dating Usually Works
At the first prenatal visit, your provider typically asks for the first day of your last menstrual period and how regular your cycles are. From that date they calculate an estimated due date by adding 280 days (40 weeks). If your cycles are regular and the date is certain, this estimate is often accepted as it stands.
How Ultrasound Confirms or Changes the Date
In the first trimester, the length of the embryo (crown-rump length) is a reliable dating measure because babies grow at a very similar pace early on. Current ACOG guidance describes adjusting the due date when the ultrasound estimate differs from the last-period estimate by more than a set number of days, and that threshold depends on how far along the pregnancy is at the time of the scan. Later in pregnancy, babies vary more in size, so ultrasound becomes a less precise dating tool and the earlier date is usually kept. See the full guidance in ACOG: Methods for Estimating the Due Date.
Special Situations
Dating works differently for irregular cycles, conception while breastfeeding or after stopping hormonal contraception, and for IVF pregnancies, where the embryo transfer date gives an exact starting point. Our article on traditional conception versus IVF timelines explains the difference.
Try It Yourself
You can estimate your own dates with the Due Date Calculator or the Gestational Age Calculator. Treat the result as an estimate and confirm it with your provider.
Why Pregnancy Is Counted From Your Last Period
Gestational age is measured from the first day of your last menstrual period (LMP), not from the day of conception. Because ovulation and conception usually occur about two weeks after the period begins, you are already considered about 2 weeks pregnant on the day you conceive. A full-term pregnancy is about 40 weeks, or 280 days, from the LMP. Counting this way keeps dating consistent, because the LMP is a date most people can recall while the moment of conception is rarely known.
How the Due Date Is Calculated: Naegele’s Rule
The traditional method, known as Naegele’s rule, adds 280 days to the first day of the LMP. In practice, that means counting forward 7 days and back 3 months. For example, an LMP of March 10 gives an estimated due date of December 17. The rule assumes a 28-day cycle with ovulation on day 14, so providers adjust the result when your cycles are shorter, longer or irregular. You can run the calculation instantly with our due date calculator or the gestational age calculator, and our LMP dating guide explains cycle adjustments in more detail.
When an Ultrasound Changes the Date
Obstetric guidance, including recommendations from the American College of Obstetricians and Gynecologists, compares the ultrasound estimate with the LMP date and keeps the LMP date unless the gap is large enough. Commonly cited thresholds are:
- Up to 8 weeks 6 days: redate if the difference is more than 5 days.
- 9 weeks 0 days to 13 weeks 6 days: redate if the difference is more than 7 days.
- 14 weeks 0 days to 15 weeks 6 days: redate if the difference is more than 7 days.
- 16 weeks 0 days to 21 weeks 6 days: redate if the difference is more than 10 days.
- 22 weeks 0 days to 27 weeks 6 days: redate if the difference is more than 14 days.
- 28 weeks and later: redate if the difference is more than 21 days.
Once the due date is set, it is generally not changed again. Read why gestational age gets adjusted for real-world examples.
Dating After IVF
With IVF, the date of fertilization or embryo transfer is known exactly, so dating is more precise. For a day-5 blastocyst transfer, the due date is typically estimated by adding 261 days to the transfer date. For a day-3 embryo transfer, add 263 days. Use the IVF due date calculator to do this automatically.
How the Weeks Map to Trimesters
- First trimester: from the start of the LMP to 13 weeks 6 days.
- Second trimester: 14 weeks 0 days to 27 weeks 6 days.
- Third trimester: 28 weeks 0 days until birth.
Term is also defined by weeks: early term is 37 to 38 weeks 6 days, full term is 39 to 40 weeks 6 days, late term is 41 to 41 weeks 6 days, and post-term is 42 weeks or later.
A Due Date Is an Estimate
Only a small share of babies, roughly 5 percent, are born on their exact due date. Most arrive within a couple of weeks either side, so think of the date as the center of a window rather than a deadline. See how accurate your due date is for more, and explore our other pregnancy calculators and the PregCalc blog to keep tracking each stage.
Related Reading
- Gestational Age FAQs — quick answers to common questions
- Traditional vs. IVF Dating — how the starting point changes
- When Gestational Age Gets Adjusted — why a due date can change
- Due Date Calculator — estimate when your baby may arrive
Also see our calculator FAQs, pregnancy FAQs and pregnancy terms glossary.
Frequently Asked Questions
How is gestational age calculated?
It is counted from the first day of your last menstrual period, not from conception. A full-term pregnancy is about 40 weeks, or 280 days, by this count.
Which is more accurate, LMP or ultrasound?
An early first-trimester ultrasound is generally the most accurate, especially when your cycles are irregular or you are unsure of your last period date.
Will my due date change after an ultrasound?
It can. If the ultrasound estimate differs from the LMP-based date by more than about five to seven days in the first trimester, providers commonly adjust the due date.
Related Resources
- Gestational Age Calculator
- Due Date Calculator
- When and Why Gestational Age Gets Adjusted
- Gestational Age vs. Fetal Age: What’s the Difference?
- The Complete Due Date Guide
Sources and Further Reading
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.