Updated June 17, 2026
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A due date is the statistical midpoint of a normal delivery window, not a firm deadline. Only about 4 to 5 percent of babies are born on their exact calculated due date, and roughly 80 percent arrive within two weeks on either side. Understanding this range can reduce anxiety and help you prepare flexibly.
Why due dates are estimates
The 40-week calculation assumes a textbook 28-day cycle with ovulation on day 14. In reality, cycle length and ovulation timing vary widely between people and even between cycles. Implantation timing also varies. All of these factors mean the true gestational age can differ from the calculated age by a week or more, even before considering individual variation in when labor begins.
How early or late do most babies arrive?
Research suggests that first-time parents tend to deliver a few days after their due date on average, while people who have given birth before often deliver slightly earlier. The full-term window runs from 37 to 42 weeks. Births before 37 weeks are considered preterm, and providers typically monitor closely or consider induction after 41 to 42 weeks.
Does ultrasound make the date more accurate?
An early first-trimester ultrasound (ideally at 8 to 12 weeks) is more accurate than an LMP-based calculation, because it measures the embryo directly rather than relying on assumed cycle timing. A first-trimester scan typically narrows the uncertainty to about plus or minus 5 to 7 days. Later scans are less precise for dating, because normal variation in fetal size widens as pregnancy progresses. If your LMP date and ultrasound date agree closely, that is a good sign the estimate is solid.
Can the due date be wrong by 2 weeks?
Yes, particularly if cycles are irregular or the LMP date is uncertain. A two-week difference is within the range of normal variation, especially without an early ultrasound to confirm. This is one reason why providers rarely induce labor or perform a caesarean based on gestational age alone when there is uncertainty about dates. See how due dates are calculated for details on the method.
What to do as your due date approaches
Prepare from about 37 weeks onward, but stay flexible. Your provider will monitor you and discuss options if you go past 41 weeks. There is no single "right" birth date; what matters most is a healthy delivery. Use the Due Date Calculator to track your current week and see how far you are from your estimated date.
Always discuss your due date, any concerns about timing, and your birth plan with your doctor or midwife.
Find your due date
Get your estimated due date and current week in seconds.