The pregnancy due date calculator and the pregnancy conception calculator handle the same calendar question from opposite ends. The due date calculator starts from a last menstrual period or conception date and estimates the delivery date, along with trimester endpoints, full term date, and current week. The conception calculator starts from an estimated due date, a last period with cycle length, or ultrasound dating and estimates when conception likely occurred, with a possible range around it. Together they cover both directions of pregnancy dating arithmetic.
What each calculator does
The pregnancy due date calculator has two modes. In last menstrual period mode, the entered date is treated as the first day of the LMP and the due date is that date plus 280 days. In conception mode, the due date is the entered conception date plus 266 days. It then builds a timeline from an internal start date — the LMP date, or conception plus 14 days — adding 84 days for the first trimester end, 182 days for the second trimester end, and 259 days for the full term date, with current week computed from the day difference between the start date and a selected current date. The page explains that clinically, gestational age is usually counted from the first day of the last menstrual period even though fertilization typically occurs later.
The pregnancy conception calculator has three methods. In due date mode, it subtracts 266 days from the entered due date. In last period mode, it estimates conception as the last period date plus (cycle length − 14) days and calculates a due date as the period date plus (cycle length − 28 + 280) days. In ultrasound mode, it converts the reported gestational age to days (7 × weeks + days), estimates conception as the scan date minus (gestational days − 14), and calculates the due date as the scan date plus (280 − gestational days). Every method returns a possible range from five days before to one day after the estimated conception date, labeled as a 7-day range.
Side-by-side
| Due date calculator | Conception calculator | |
|---|---|---|
| Direction | Forward: date in, delivery date out | Backward: due date or period in, conception date out |
| Modes | LMP (+280 days) or conception (+266 days) | Due date (−266 days), last period with cycle length, or ultrasound |
| Extra outputs | Trimester ends, full term date, current week | Calculated due date, possible 7-day conception range |
| Key convention | 40 weeks from LMP; 38 weeks from conception | Cycle length − 14 days estimates ovulation |
| Range handling | Single estimated date, described as an estimate | Single estimate plus a 7-day window reflecting sperm survival and ovulation uncertainty |
| Clinical framing | Clinician-assigned EDD takes priority | Not proof of paternity, ovulation, or a specific intercourse date |
When to use which
Use the due date calculator when planning forward from a known date: the first day of the last menstrual period, or a conception date. It gives the estimated due date plus the trimester and full-term milestones used for planning, and its page notes that related tools include the pregnancy conception calculator and the reverse due date calculator.
Use the conception calculator when working backward: you have an estimated due date, a last period date with cycle length, or an ultrasound gestational age, and you want a likely conception window. The page recommends it for education and planning, and stresses that a clinically assigned due date or early ultrasound is often more useful than memory of cycle timing, while last period dating can be less reliable with irregular cycles, recent hormonal contraception, breastfeeding, or uncertain bleeding dates.
Limits and disclaimer
Both pages are educational date arithmetic, not medical advice. The due date page warns that due dates are based on population averages, that cycle length, ovulation timing, uncertain dates, ultrasound measurements, and assisted reproduction can shift dating, and that the current conception-mode timeline start (conception plus 14 days) makes trimester endpoints and current week later than typical gestational-age counting would suggest. The conception page warns that the result is not proof of paternity, ovulation, or a specific intercourse date, and that the ultrasound formula can place conception after the scan date for very early scan ages — a formula edge case to discuss clinically. Neither page evaluates fetal growth, symptoms, or viability; consult an obstetric clinician for pregnancy dating and care.