Your due date is most commonly estimated by counting 280 days (40 weeks) from the first day of your last menstrual period. If your periods are irregular or you don’t know that date, an ultrasound measurement taken in the first trimester can provide a more accurate estimate. Remember that a due date is exactly that — an estimate. Only about 1 in 20 babies arrives on their due date.
How To Estimate Your Due Date Using Your Last Menstrual Period (LMP)
This is the standard method doctors have used for decades. It is called Naegele’s rule. You start with the first day of your last menstrual period. Then add 7 days, subtract 3 months, and add 1 year. For example, if your LMP started on June 1, you add 7 days to get June 8, subtract 3 months to get March 8, then add 1 year. Your estimated due date would be March 8 of the following year.
This method assumes a regular 28-day cycle and that ovulation occurred on day 14. Most women do not have a perfect 28-day cycle. If your cycle is longer or shorter, the due date from LMP may be off by several days. Even so, LMP is still the first calculation your healthcare provider will make.
You can also count ahead 40 weeks from the first day of your LMP. Many pregnancy apps and online calculators do this automatically. They are using the same rule. The LMP method works well for women with regular cycles who are certain of the date.
How Ultrasound Helps Estimate Your Due Date
Ultrasound dating is the most accurate method, especially in the first trimester. Between 8 and 13 weeks of pregnancy, a technician measures the baby from crown to rump — the top of the head to the bottom of the buttocks. This measurement, called the crown-rump length, correlates closely with gestational age.
First-trimester ultrasound dating is accurate to within about 5 to 7 days. This is because early fetal growth follows a very consistent pattern. Later in pregnancy, individual growth differences make ultrasound dating less precise. A second-trimester ultrasound is accurate to about 10 to 14 days. A third-trimester ultrasound can be off by 3 weeks or more.
If your LMP and ultrasound due dates differ by more than a week in the first trimester, most providers will use the ultrasound date. This is especially important for determining your due date when you have irregular periods or when the LMP date is unknown.
What If You Have Irregular Periods or Don’t Know Your LMP?
Irregular cycles make LMP-based dating unreliable. If you have polycystic ovary syndrome, are breastfeeding, or recently stopped birth control, your ovulation may not follow a predictable schedule. In these cases, ultrasound is the preferred method.
If you conceived while using hormonal birth control or after a miscarriage, you may not have a clear LMP date at all. Your healthcare provider will schedule an early ultrasound, ideally between 8 and 10 weeks, to estimate the due date. The earlier the ultrasound, the more reliable the date.
Some women track ovulation using basal body temperature or ovulation predictor kits. If you know the exact date of ovulation, you can add 266 days (38 weeks) to estimate your due date. This is biologically more accurate because gestation is actually about 266 days from conception. But most providers still use LMP as a starting point because not every woman tracks ovulation.
Why Your Due Date Is Just an Estimate
It is important to understand that a due date is a target, not a deadline. A full-term pregnancy ranges from 37 weeks to 42 weeks. Babies born at 37 weeks are considered early term, but most are healthy. Babies born between 39 and 40 weeks have the lowest rates of complications. Pregnancies that last 41 or 42 weeks are called late term and post term.
Only about 5 percent of babies are born on their exact due date. Most arrive in the two weeks before or after that date. The due date helps your provider schedule important tests, such as blood work, glucose screening, and growth ultrasounds. It also guides decisions about inducing labor if you go past 41 or 42 weeks.
Knowing that the due date is an estimate can reduce anxiety. Many first-time parents worry when labor does not start on the exact day. Understanding the normal range of term pregnancy can help you feel more prepared.
How Accurate Are Due Date Calculations?
LMP-based due dates are reasonably accurate for women with regular 28-day cycles who are sure of their LMP. Research shows that LMP dating agrees with first-trimester ultrasound dating within about one week for most women. But when cycles are irregular, LMP can overestimate or underestimate gestational age by several weeks.
First-trimester ultrasound is considered the gold standard for dating. Studies have found that assigning a due date based on a first-trimester scan reduces the number of pregnancies labeled post-term and lowers the rate of unnecessary inductions. The American College of Obstetricians and Gynecologists recommends that if there is a discrepancy of more than 7 days between LMP and first-trimester ultrasound, the ultrasound date should be used.
No method is perfect. All due date calculations assume a consistent rate of early fetal growth. In rare cases, very early ultrasound measurements may be off by a few days due to operator technique or fetal position. But overall, first-trimester ultrasound is the most reliable tool available.
What Happens If You Go Past Your Due Date?
If you reach 40 weeks and have not gone into labor, your provider will monitor you more closely. This typically involves extra checkups, nonstress tests, or ultrasound to check the baby’s size and amniotic fluid levels. These tests are done to make sure the baby remains healthy.
Most providers recommend induction of labor between 41 and 42 weeks. Waiting longer than 42 weeks is associated with a slightly increased risk of stillbirth and other complications. The exact timing depends on your health, the baby’s condition, and your preferences. Some women choose to wait a few days past 41 weeks; others prefer induction earlier.
If you are determined to avoid induction, ask your provider about membrane sweeping — a simple office procedure that can nudge labor to start naturally. But induction is common and safe when done for medical reasons. Your provider will explain the risks and benefits based on your specific situation.
Can You Do Anything To Influence Your Due Date?
Your due date is set by the size and growth of your baby, not by anything you do. No evidence shows that certain foods, exercises, or sexual activity can bring on labor or change your due date. Some folk remedies, such as eating spicy food or walking, may be harmless but have no proven effect.
Pregnancy length is influenced by genetics and biological factors. For example, women who were themselves born at 40 weeks tend to deliver their own babies at a similar time. First-time pregnancies often last slightly longer than subsequent ones. These are general trends, not rules.
Focus on maintaining a healthy pregnancy rather than trying to shift your due date. Eat well, stay active, attend all prenatal appointments, and follow your provider’s advice about when to come in for labor. Your baby will arrive on their own schedule.
Frequently Asked Questions
How is my due date calculated?
Your provider calculates it by adding 280 days (40 weeks) to the first day of your last menstrual period. If your periods are irregular, your due date is set using an early ultrasound measurement.
Can my due date change?
Yes, especially if an early ultrasound shows a difference of more than one week from your LMP-based date. After the first trimester, your due date is usually not changed because later scans are less accurate.
What if my baby comes before the due date?
Babies born between 37 and 38 weeks are considered early term and are generally healthy. If you go into labor before 37 weeks, that is preterm, and your baby may need extra medical care.
How accurate is ultrasound dating?
First-trimester ultrasound dating is accurate to within about 5 to 7 days. Second-trimester scans are accurate to about 10 to 14 days. Later scans become less precise due to natural variations in fetal growth.
