Your due date is an estimate, not an appointment. Pregnancy lasts about 40 weeks from the first day of your last menstrual period, and there are three common ways to calculate it: counting from your last period, using the date of conception or ovulation, and using an early ultrasound measurement. Each method works best in different situations, and they can give you slightly different dates.
The method your clinician uses depends mostly on how certain you are about your cycle and how early in pregnancy you are seen. Early ultrasound is generally the most accurate when it is available. The other two methods are useful when it is not.
How To Figure Out Your Due Date: 3 Methods
All three methods aim at the same target: the day you reach 40 weeks of gestation. They just start counting from different points.
Method 1: Last menstrual period (LMP). This is the classic approach. You count 280 days (40 weeks) forward from the first day of your last period. A common shortcut is Naegele’s rule: add one year to your last period date, subtract three months, then add seven days.
This method assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, or if you ovulate at a different point, the estimate shifts. That is one reason LMP dating is less reliable for people with irregular cycles.
Method 2: Conception or ovulation date. If you know when you ovulated or had intercourse that led to conception, you can count 266 days (38 weeks) forward from that date. This is essentially the same as the LMP method minus two weeks, because the LMP method includes the roughly two weeks before ovulation.
This method is accurate when the date is genuinely known, such as with tracked ovulation or a single known conception date. Many people do not know this date precisely, so it is used less often than the other two.
Method 3: Ultrasound. An early ultrasound measures the embryo or fetus and estimates gestational age from its size. In the first trimester, crown-rump length is used. Later, measurements like head circumference, abdominal circumference, and femur length are combined.
Ultrasound is generally the most accurate method in early pregnancy, when size differences between normally developing babies are small. The earlier the scan, the more reliable the dating.
Why Does the Method Matter So Much?
The method matters because your due date is not just a number on a calendar. It shapes decisions throughout pregnancy.
Gestational age affects when certain screening tests are offered, when growth is assessed, and when a pregnancy is considered term or post-term. If the dating is off by a week or more, those decisions can shift.
That is why professional guidance generally favors early ultrasound dating when it is available. An accurate date reduces the chance of unnecessary interventions later, and it reduces the chance of missing a problem because the timing was miscalculated.
Here is a detail many people miss: a due date is a midpoint, not a deadline. Only a small share of babies arrive on their estimated date. Most are born within a window of a few weeks around it.
How Accurate Is Each Method?
Accuracy depends on how early the information is gathered and how certain the underlying date is.
- LMP: Reasonably accurate for people with regular 28-day cycles. Less reliable with irregular cycles, recent hormonal changes, or uncertain dates.
- Conception date: Accurate when the date is truly known. Often unavailable or uncertain.
- Early ultrasound: Generally the most accurate, especially in the first trimester. Accuracy decreases as pregnancy advances.
When methods disagree, clinicians usually rely on the earliest reliable measurement. An early ultrasound typically takes precedence over LMP dating because it is based on the pregnancy itself rather than on a recalled cycle.
No method is perfect. Even a first-trimester ultrasound has a margin of error of several days. That uncertainty is normal and expected.
What Counts as Full Term?
Pregnancy is divided into ranges rather than a single cutoff. These ranges come from standard obstetric definitions.
| Category | Weeks |
|---|---|
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post term | 42 weeks 0 days and beyond |
These ranges replaced the older idea that anything from 37 to 42 weeks was simply “term.” Research showed that outcomes differ across that span, which is why the more specific categories were adopted.
This is one reason accurate dating matters. If your date is off, you might be treated as full term when you are actually early term, or the reverse.
Can Your Due Date Change During Pregnancy?
Yes. A due date can be adjusted, usually after an early ultrasound.
If a scan measures the pregnancy at a size that differs from the LMP estimate by more than a set amount, the clinician may revise the date to match the ultrasound. The threshold for changing the date is smaller in early pregnancy and larger later on, because normal size variation grows as pregnancy progresses.
After the first trimester, dating is less likely to be changed based on size alone. Later growth reflects many factors beyond gestational age, so it is a weaker dating tool.
Once a reliable date is set, it usually stays fixed for the rest of the pregnancy. That fixed date becomes the reference point for everything that follows.
What If You Do Not Know Your Last Period?
This is common, and it is not a problem. Many people have irregular cycles, recent birth control use, or simply do not recall the date.
In these cases, an early ultrasound becomes the primary dating method. It does not depend on your memory of a cycle, only on the measurement of the pregnancy itself.
If no early ultrasound is available, clinicians use the best information they have and may rely on later scans, though those are less precise for dating. Physical exam findings alone are not a reliable way to establish gestational age.
The takeaway is simple: uncertainty about your last period is normal, and there is a standard path forward. An early scan usually resolves it.
Does a Due Date Predict When You Will Deliver?
No. A due date estimates gestational age. It does not predict the day you will go into labor.
Labor onset depends on many factors, and only a minority of people deliver on their estimated date. Most deliver within a few weeks before or after it.
What the due date does is provide a consistent reference for monitoring and decision-making. It helps clinicians track growth, time screenings, and recognize when a pregnancy has gone past the expected range.
Think of it as a measuring tool, not a forecast. It tells you where you are in pregnancy, not exactly when it will end.
Frequently Asked Questions
How do you calculate your due date from your last period?
Count 280 days forward from the first day of your last menstrual period. A common shortcut is to add one year, subtract three months, and add seven days.
Is ultrasound more accurate than counting from your last period?
Early ultrasound is generally more accurate, especially in the first trimester. It measures the pregnancy directly instead of relying on a recalled cycle date.
Can my due date change after an early ultrasound?
Yes, it can be adjusted if the scan measurement differs enough from the last-period estimate. The threshold for changing it is smaller early in pregnancy and larger later.
What if I do not know the date of my last period?
An early ultrasound is usually used to establish the date instead. Physical exam findings alone are not a reliable way to determine gestational age.
