Understanding Your Due Date
A due date gives you and your care team a shared timeline for prenatal appointments, screenings, and birth planning. Understanding how it's calculated and what to expect along the way helps you feel prepared at each stage.
How is a due date calculated?
The standard method is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle and ovulation on day 14. If your cycle is longer or shorter, the due date shifts accordingly — which is why this calculator lets you enter your actual cycle length. Your doctor may also adjust the date after an early ultrasound, which measures the size of the embryo and is often more accurate than date-based estimates.
How accurate is a due date calculator?
Only about 5% of babies are born on their exact due date. Most arrive within two weeks before or after — the normal range is 37–42 weeks. A first-trimester ultrasound (ideally before 14 weeks) is the most accurate way to date a pregnancy. The calculator here gives you a reliable estimate, but your healthcare provider's assessment — especially from an early scan — takes precedence.
What week is the anatomy scan?
The anatomy scan (also called the 20-week scan or mid-pregnancy ultrasound) is typically scheduled between 18 and 22 weeks, most commonly right around 20 weeks. During the scan, a sonographer checks fetal anatomy including the brain, heart, spine, kidneys, and limbs, measures growth, checks placenta position, and can often determine fetal sex if you choose to find out.
Frequently asked questions
How is a pregnancy due date calculated?
The most common method (Naegele's rule) adds 280 days (40 weeks) to the first day of your last menstrual period. If you know your conception date, add 266 days. Your doctor may adjust the date based on an early ultrasound, which is generally more accurate than calendar-based methods.
What week does each trimester start?
The first trimester runs from week 1 through week 12. The second trimester is weeks 13–27. The third trimester begins at week 28 and continues until delivery, typically around week 40. Key screenings, scans, and symptoms differ by trimester, so knowing which one you're in helps you know what to expect next.
Can I find out the baby's sex at the anatomy scan?
The anatomy scan (typically at 20 weeks) can often reveal fetal sex if the baby is in a favorable position and you choose to find out. Some parents ask the technician not to reveal it, or to write it in an envelope for a gender reveal. Early blood tests like NIPT (around 10 weeks) can also detect fetal sex with high accuracy.