The Number Everyone Asks For
The moment you share your news, everyone wants the date. You will type it into forms, build countdowns around it, and maybe field daily texts as it approaches. So it helps to know the honest truth up front: only about 4 to 5 percent of babies arrive on their due date. The date is a well-informed estimate of the middle of a window, not an appointment.
How the Date Gets Calculated
The classic method, called Naegele's rule, starts from the first day of your last menstrual period and adds 280 days, or 40 weeks. Our due date calculator does this math instantly, and can also work from a conception date or IVF transfer date, which are more precise starting points when you have them.
The catch with the calendar method: it assumes a 28-day cycle with ovulation on day 14. Real cycles vary, which is why the date often moves.
Why Ultrasound Usually Wins
A first-trimester ultrasound measures the embryo directly, and early growth is remarkably uniform from pregnancy to pregnancy. Between roughly 8 and 13 weeks, that measurement dates a pregnancy within about 5 to 7 days, which is why providers will often adjust your original date after the first scan. If your dates move a few days at that appointment, nothing is wrong; the estimate just got better.
For IVF pregnancies, the transfer date makes dating the most precise of all, and the date rarely changes.
The Real Window
Full term is a range, not a day. Most babies arrive between 37 and 42 weeks, and first babies run slightly later on average. A more honest way to answer "when are you due?" is a two-week window around your date. Some parents share only the month for exactly this reason, and it is a wise move for the group chat's collective blood pressure.
What the Date Is Actually For
Even though the day itself is a soft target, the date anchors real decisions: the timing of screenings and scans, gestational diabetes testing, when a pregnancy counts as term, and conversations about induction if you pass 41 weeks. So the estimate matters; the calendar-square certainty does not.
Make the Window Work for You
- Get your date from the due date calculator, then let your provider refine it after the first ultrasound
- Plan readiness for 37 weeks, not 40: bag packed with the hospital bag checklist, car seat installed, birth plan drafted in the birth plan builder
- Build your team well before the window opens. Doulas book up months ahead, and you can find birth support near you in the directory.
Your Due Date, Held Loosely
Your due date is the midpoint of a window, sharpened by an early ultrasound and still soft by a week or two in either direction. Hold it loosely, prepare for the early edge, and let the countdown be a range.