Estimate a pregnancy due date starting from a known conception date, rather than last period date.
These calculators use established formulas (such as Mifflin-St Jeor for calorie needs or the Devine formula for ideal weight) to provide general estimates. They're built for a general population and don't account for individual medical conditions, medications, or circumstances. They aren't a substitute for advice from a doctor, registered dietitian, or other qualified healthcare provider — especially for anything related to pregnancy, chronic conditions, or medication dosing.
This method adds 266 days (38 weeks) directly to a known conception date, rather than the more common approach of adding 280 days (40 weeks) to the first day of your last menstrual period.
The two methods are actually calculating the same underlying due date, just from different starting points — 280 days from LMP and 266 days from conception typically land close to the same date, since ovulation (and likely conception) usually falls around day 14 of a standard cycle.
This calculator is most useful for people who know their conception date with more confidence than their last period date — for example, from fertility tracking, IVF, or a known single date of intercourse.
A conception date of January 1 gives an estimated due date of September 24 — 266 days later.
280 days is counted from the last menstrual period, which is typically about 14 days before conception for a standard cycle — 266 days from conception lands on approximately the same calendar date.
If you know your conception date confidently (from IVF timing or fertility tracking, for example), this method can be more precise than an LMP estimate on an irregular cycle. For most pregnancies, an early ultrasound remains the most accurate dating method overall.
Use the LMP-based due date calculator instead, or rely on your provider's ultrasound dating, which doesn't depend on knowing either date precisely.