The starting date decides the due-date method
The Due Date Calculator offers three starting points: first day of the last menstrual period, conception date, or IVF embryo transfer date. Each mode uses a different date relationship to estimate an expected delivery date, so the most accurate input is the one that matches the pregnancy record already available.
A due date is a planning estimate. Clinical dating from a healthcare professional should be followed when it differs from simple calculator arithmetic.
Last-period mode adds two hundred eighty days
When the first day of the last menstrual period is entered, the calculator adds 280 days. That equals 40 weeks, a common pregnancy-dating convention used to build an estimated due date from the LMP start.
This mode assumes the last period date is known and that the pregnancy timeline follows the usual relationship between LMP and ovulation. Irregular cycles can make that assumption less precise.
Conception mode counts from fertilization timing
When a conception date is entered, the calculator adds 266 days. That is about two weeks shorter than the LMP method because conception usually occurs after the period begins.
This mode is useful when a conception date is medically estimated or strongly known. It should not be treated as proof of an exact biological moment when the date is only guessed.
IVF mode adjusts for embryo age
The IVF option asks for embryo transfer date and whether the embryo was 3 days or 5 days old at transfer. The calculator subtracts the embryo age from the conception-based count so the due date reflects the development already completed before transfer.
For IVF care, clinic records should be the primary source. Use this page as a date-checking aid, not as a replacement for the fertility clinic timeline.
Due dates are not delivery promises
The calculated date is an expected due date, not a prediction that birth will happen on that exact calendar day. Many pregnancies deliver before or after the due date. The result is most useful as a scheduling anchor for appointments, planning, and week-by-week tracking.
If a clinician changes the assigned due date after ultrasound or record review, use the updated clinical date consistently.
Current pregnancy week belongs on the timeline page
This page focuses on estimating the due date from a known starting point. If the question is current pregnancy week, trimester, or how far along the pregnancy is today, the Pregnancy Calculator is the better page.
Conception estimates need careful wording
A due date can be used to estimate likely conception timing, but that estimate is not exact. Ovulation timing, cycle length, sperm survival, ultrasound dating, and IVF records can all shift interpretation.
For that specific backward calculation, the Pregnancy Conception Calculator keeps the focus on likely conception timing.
Date entry mistakes can move the result by weeks
A wrong month, day, year, or mode selection can create a due date that looks valid while being completely off. Before using the result, confirm whether the entered date was LMP, conception, transfer date, or already an estimated due date.
This is especially important when copying dates from messages, screenshots, clinic portals, or handwritten notes.