How it works
This tool takes a confirmed delivery expectation and counts backward to reveal the likely LMP and fertilization timeframe. It is built as a multi-method comparison: you can see your results derived from menstrual history, a clinical ultrasound scan, and an IVF embryo transfer date displayed side by side, so you can evaluate which baseline is most dependable for your situation.
Clinicians establish a delivery expectation using a 280-day gestational span. Because egg release typically happens 14 days into that span, the period between fertilization and birth is about 266 days. This tool reverses that arithmetic: feed in a delivery expectation, and it outputs the probable fertilization moment and the inferred start of your final menstrual bleeding.
The formula
LMP = Due Date − 280 days; Conception = Due Date − 266 days
Worked example
Consider a patient holding a target delivery expectation of 2025-09-20. She wants to evaluate three different clinical data points to see how closely they align.
Method 1 — Menstrual history (Naegele's rule):
LMP from menstrual history: 2025-09-20 − 280 days = 2024-12-14
Fertilization from menstrual history: 2025-09-20 − 266 days = 2024-12-28
Method 2 — Early ultrasound scan: An 8-week dating scan placed the delivery expectation at 2025-09-15 instead, a five-day variance from the cycle-based figure.
LMP from ultrasound scan: 2025-09-15 − 280 days = 2024-12-09
Fertilization from ultrasound scan: 2025-09-15 − 266 days = 2024-12-23
Method 3 — IVF embryo transfer: A 5-day blastocyst was transferred on 2024-12-23. The clinic adds 261 days to a Day 5 transfer to reach the delivery expectation, yielding 2025-09-10.
IVF delivery expectation: 2024-12-23 + 261 days = 2025-09-10
LMP inferred from IVF transfer: 2025-09-10 − 280 days = 2024-12-13
Fertilization inferred from IVF transfer: 2025-09-10 − 266 days = 2024-12-27
| Method | Delivery Expectation | Inferred LMP | Probable Fertilization |
|---|---|---|---|
| Menstrual history | 2025-09-20 | 2024-12-14 | 2024-12-28 |
| Ultrasound scan | 2025-09-15 | 2024-12-09 | 2024-12-23 |
| IVF transfer | 2025-09-10 | 2024-12-13 | 2024-12-27 |
The three methods cluster the probable fertilization moment between December 23 and December 28. For IVF patients, the transfer-based arithmetic is generally the most precise, since the embryo's developmental stage is documented to the day.
Things to watch
A single day's difference in the source data can shift the inferred fertilization window by a full day. First-trimester ultrasound measurements carry the highest clinical weight because early fetal sizing follows a highly uniform trajectory, whereas menstrual recall relies on patient memory and assumes a textbook 28-day cycle. If your cycles run longer or shorter than 28 days, the cycle-based estimate will drift proportionally.
Sperm can persist for up to five days inside the reproductive tract, and the released egg remains viable for roughly 24 hours. That biology creates a six-day fecundability corridor, meaning intercourse anywhere in that span could be responsible for the pregnancy. Pinpointing a single calendar day of fertilization is not biologically possible from arithmetic alone — the result is always a window, not a certitude.
These outputs are an estimate, not professional medical advice. Your obstetrician or midwife integrates scan findings, clinical history, and physical measurements to finalize your official prenatal schedule.