How pregnancy weeks and due dates are calculated

Pregnancy is counted from the first day of your last menstrual period, not from the day you conceived. This means you are considered two weeks pregnant before conception even happens. Your due date is set at 40 weeks from that first day of your last period — roughly 280 days later.

Doctors use this method because most people do not know the exact day they conceived, but they usually know when their last period started. The calculation is straightforward: count back three months from the first day of your last period, then add one year and seven days. That gives you your estimated due date.

Your actual delivery may happen anywhere from two weeks before to two weeks after this date. Only about 5 percent of pregnancies deliver on the exact due date. The due date is a target, not a may provide.

Key Takeaways

  • Pregnancy is counted from the first day of your last menstrual period, making you two weeks pregnant before conception occurs.
  • Your due date is 40 weeks from the first day of your last period, calculated by counting back three months and adding one year and seven days.
  • Ultrasound measurements in the first trimester can adjust your due date by a few days if your cycle is irregular or dates are uncertain.
  • Delivery typically occurs between two weeks before and two weeks after your due date, and only a small percentage of pregnancies deliver on the exact date.
  • Gestational age (how far along you are) is different from fetal age (how old the fetus is), with gestational age being two weeks ahead.

The three-month rule for quick calculation

The fastest way to find your due date by hand is the three-month rule. Take the first day of your last menstrual period and count backward three calendar months. Then add seven days.

For example: if your last period started on March 15, count back three months to December 15, then add seven days. Your due date would be December 22. This method works because it accounts for the roughly 280-day pregnancy length in a way that aligns with how months vary in length.

This calculation assumes a standard 28-day cycle. If your cycle is longer or shorter, your actual conception date shifts, which means your due date may shift too. An ultrasound in the first trimester can correct for this.

Using an ultrasound to confirm or adjust your date

An ultrasound in the first trimester — ideally between 8 and 14 weeks — is the most accurate way to confirm your due date. The technician measures the fetus from crown to rump and compares it to standard growth charts. If the measurement does not match your calculated date, your due date may be adjusted by a few days.

Ultrasounds in the second and third trimesters are less precise for dating because fetal growth varies more widely by that point. A second-trimester ultrasound can be off by up to two weeks. A third-trimester ultrasound can be off by three weeks or more.

If you have an early ultrasound and it shows a different due date than your period-based calculation, your doctor will usually use the ultrasound date as the new reference point. This is especially important if the difference is more than a few days.

Understanding gestational age versus fetal age

Gestational age is how far along your pregnancy is, counted from the first day of your last period. Fetal age is how old the fetus actually is from conception. Gestational age is always about two weeks ahead of fetal age because of how the counting system works.

When your doctor says you are 8 weeks pregnant, they mean 8 weeks of gestational age. The fetus is actually about 6 weeks old. This difference matters for understanding growth milestones and when certain tests are done, but it does not change your due date.

Pregnancy milestones — like when the heartbeat becomes detectable or when you might feel movement — are tied to fetal age, not gestational age. Knowing both numbers helps you understand what to expect at each stage.

Adjusting your due date if you are unsure of your last period

If you do not remember the exact first day of your last period, your doctor can estimate it based on an early ultrasound or on when you first felt fetal movement. Quickening — the sensation of the fetus moving — typically happens around 16 to 20 weeks of pregnancy, though it can vary.

An ultrasound is more reliable than movement for dating purposes. If you are already in your second trimester and have no ultrasound from the first trimester, a second-trimester scan can estimate your due date, but the margin of error is larger — usually within two weeks either way.

If you have irregular periods or use hormonal birth control, your cycle length may not be the standard 28 days. Tell your doctor about your typical cycle length so they can adjust the calculation or rely more heavily on ultrasound dating.

Timeline markers from conception to due date

Pregnancy unfolds in predictable stages once you know your due date. The first trimester runs from week 1 to week 13 and includes the formation of major organs. The second trimester, weeks 14 to 27, is when the fetus grows rapidly and movement becomes noticeable. The third trimester, weeks 28 to 40, focuses on weight gain and preparation for birth.

Key milestones include a detectable heartbeat around 6 to 7 weeks of gestational age (4 to 5 weeks fetal age), visible movement on ultrasound around 8 to 9 weeks, and quickening felt by the pregnant person around 16 to 20 weeks. Viability — the point at which a fetus could survive outside the womb with medical support — is generally considered to be around 22 to 24 weeks.

Your due date marks the end of the 40-week pregnancy. Pregnancies that go past 42 weeks are considered post-term and carry increased risks, so induction is usually offered at that point.

Frequently Asked Questions

What if my due date changes after an ultrasound?

Due date changes are common, especially if your first ultrasound is in the first trimester. Your doctor will use the ultrasound date if it differs significantly from your period-based calculation. This does not mean something is wrong — it usually just means your cycle was longer or shorter than average, or you ovulated later than expected.

Can I calculate my due date if I use birth control or have irregular periods?

The three-month rule is less reliable if your cycle is not regular. An early ultrasound becomes more important for accurate dating. Tell your doctor about your typical cycle length and when you stopped using birth control so they can adjust their estimate accordingly.

Is my baby late if I deliver after my due date?

No. Delivery between 39 and 40 weeks is considered full-term. Delivery between 37 and 38 weeks is early-term but still normal. Your due date is an estimate, and most pregnancies deliver within two weeks of it. Your doctor will monitor you if you go past 42 weeks.

Does the three-month rule work for all pregnancies?

The three-month rule assumes a 28-day cycle and ovulation on day 14. If your cycle is 30 days or 26 days, the calculation shifts slightly. An ultrasound in the first trimester is the most accurate way to confirm your due date regardless of your cycle length.

When should I have my first ultrasound for dating?

The best time for an accurate dating ultrasound is between 8 and 14 weeks of pregnancy. If you have an ultrasound before 8 weeks, it can still date your pregnancy, but the margin of error is slightly larger. After 14 weeks, ultrasound dating becomes progressively less precise.