Pregnancy due date calculator

Most people know when their last period started, but not exactly when they ovulated or conceived. Using LMP as a reference gives a consistent starting point. It's an old convention (dating back to the 1800s), but it's standardized worldwide. Doctors adjust it with ultrasound if needed.

Doctors and midwives refer to pregnancy as "12 weeks 3 days" or "36+5" (36 weeks, 5 days). This precision matters for medical milestones—viability, testing windows, and term classifications. "Second trimester" is weeks 13–26, "third trimester" is weeks 27–40, but specific weeks are more useful clinically.

Early term: 37–38 weeks. Full term: 39–40 weeks. Late term: 41 weeks. Post-term: 42+ weeks. Babies born at 37 weeks are full-term and generally healthy, but those born at 39–40 weeks have slightly better outcomes on average. Your provider monitors you closely as you approach and pass your due date.

Your doctor will use ultrasound to estimate gestational age, usually during your first prenatal visit. Early ultrasound (before 13 weeks) is the most accurate way to date a pregnancy. Tell your provider if your cycles are irregular or if you're unsure about LMP.

LMP-based dating is accurate to within 1–2 weeks if you have regular cycles. Early ultrasound is more accurate (within 3–5 days). As pregnancy progresses, growth varies more between babies, so later ultrasounds are less useful for dating. Most due dates are estimates within a week or so of the actual delivery.

Your provider might adjust your due date based on ultrasound measurements, especially if there's a big discrepancy with LMP dating. Once set (usually after the first ultrasound), it typically doesn't change unless there's a clear reason. Due dates aren't moved lightly.

Your provider will monitor you more closely. Most induce labor by 41 or 42 weeks to reduce risks. Many people go into labor naturally before then. Going a week or two past your due date is common and usually not a concern as long as you and baby are healthy.