Best Practices for Pregnancy Due Date Prediction

A due date is only as trustworthy as the information that feeds it. Naegele's rule is mathematically trivial — add 280 days — but the inputs are anything but. These best practices cover the habits that produce the most reliable estimate: accurate recall of the last menstrual period, honest treatment of cycle length, and knowing when to hand the calendar over to an ultrasound.

Pin down the first day of the last menstrual period with precision. The LMP is not the day bleeding ended or the day you first suspected pregnancy; it is the calendar day the most recent period actually started. Because the entire estimate pivots on that single date, a two-day recall error shifts the due date by two days, and a one-week error shifts it by a week. Write the date down early — ideally the day the period begins — rather than reconstructing it from memory weeks later.

Be honest about cycle length. Naegele's rule assumes a 28-day cycle with ovulation on day 14, but real cycles range from roughly 21 to 35 days and ovulation can occur later than day 14. A woman with a 35-day cycle who ovulates around day 21 is typically about a week less far along than the LMP math suggests. If your cycles are consistently longer or shorter than 28 days, mention it to your provider — and recognize that the conception-date option, when ovulation is genuinely tracked, will usually be the more accurate of the two routes.

Use the conception path only when the date is real, not guessed. Entering "conception" as the day you think you might have conceived, or using the date of an encounter that was not actually ovulation, merely relocates the guess. The calculator adds 266 days to whatever you type, so garbage in is garbage out. If ovulation was tracked with temperature or LH strips, the conception date is defensible; otherwise the LMP route is the safer default.

Treat the early ultrasound as the correction tool it is designed to be. Obstetric guidance in many countries recommends a dating ultrasound in the first trimester, typically around 8 to 13 weeks, measuring the crown-rump length. Because fetal size is remarkably consistent in early pregnancy, this measurement can refine the due date to within a few days — and, when the LMP is unknown or cycles are irregular, it often becomes the authoritative date. If the ultrasound-based and LMP-based dates differ by more than a few days, follow the ultrasound-based date.

Recompute gestational age the right way at every milestone. Gestational age is always counted in completed weeks and days from the LMP, and the calculator handles this automatically for any current date you enter. When discussing your pregnancy, "weeks" means completed weeks — at 13 weeks 6 days you are in your 14th week, and the trimester label flips only when you complete week 27 and enter week 28. Keeping this convention straight avoids confusion when coordinating scans and appointments.

Use the milestone table as a screening-calendar, not a source of anxiety. The milestones listed — first-trimester screening, anatomy scan, viability, full term — are typical clinical checkpoints, and the exact week a center schedules them varies. If a milestone shows "Upcoming" when your provider is already planning that test, trust the provider. The tool is an orientation aid; your care team's schedule is the operational truth.

Update your records after every scan. The due date that matters for labor planning may change after a dating ultrasound, and the current date you enter into the calculator should always reflect where you are now, not where you were at your last visit. Keeping a single written record — LMP, conception date if known, ultrasound-adjusted EDD, and today's gestational age — prevents the small arithmetic inconsistencies that accumulate over nine months of appointments.

Finally, communicate in the language your provider uses. When you share your due date, also share the LMP it was built from, because that is the reference your care team will use to compute gestational age themselves. The Pregnancy Due Date Predictor runs entirely in your browser and never stores or transmits your dates, so you can use it freely alongside your official records. Combine accurate recall, honest cycle information, and ultrasound confirmation, and the estimated date you plan around will be the most reliable one modern obstetrics can offer.