Pregnancy dates and due dates

The page provides four separate modes: first day of the last menstrual period (LMP), known estimated or assigned due date, known or assumed conception date, and IVF/embryo-transfer date. Every mode needs its start date and an `As of` date. The latter determines the date for which weeks, trimester and remaining days are shown.

All dates must be real ISO dates in `YYYY-MM-DD` format. LMP also requires an integer cycle length from 20 to 45 days. IVF accepts only embryo age 3 or 5 days. Other entries do not produce a service result.

The modes are different calculation starting points. The tool can derive a shared timeline from an entered value, but cannot establish how that value was medically determined.

Whatever the mode, the service normalises the calculation to an estimated due date (EDD), a calculated gestational start, and an estimated conception date. It also derives the start of the second and third trimesters and, where applicable, the transfer date and EDD in the timeline.

The date engine validates real calendar dates strictly and calculates with UTC midnight and whole days, not local time. Daylight-saving changes, month ends and leap years therefore do not shift outputs by hours; invalid dates are rejected rather than adjusted automatically.

The `W+D` notation, for example `12+6`, is shown only from the calculated start through 42+0. Before that start and substantially after 42+0, the service leaves pregnancy week and trimester empty rather than presenting a running value. On the EDD itself, 280 days since gestational start are `40+0`.

The first entered LMP day is the calculated gestational start. With a 28-day cycle the model adds 280 days; a longer or shorter entered cycle shifts only this calendar estimate by its difference from 28 days. The service then derives estimated conception as EDD minus 266 days.

Only LMP produces estimated ovulation and its possible window in the current tool. Both are back-calculated from the entered cycle model, not observations of a biological event or a reliable statement about individual fertility or contraception.

In `Known due date`, the entered value remains the EDD. The calculator works backwards and uses no cycle length, ovulation estimate or fertile window. It does not check how the date was established and cannot medically confirm or correct it.

This mode also processes only its entered calendar date. The resulting conception date is a model starting point and cannot prove a biological time. Cycle length or IVF embryo age has no calculation effect in these two modes.

IVF accepts day 3 or day 5 only and additionally displays transfer date and selected embryo age. Unlike LMP, this rule has no cycle correction and no ovulation or window line. It describes service day offsets only, not treatment, likelihood of success or medical assessment.

Enter LMP `2026-01-01`, cycle length 30 and `As of` `2026-04-01`. The service calculates EDD = `2026-01-01` + 280 + (30 − 28) days = `2026-10-10`. It returns gestational start `2026-01-01`, estimated conception and ovulation `2026-01-17`, and possible window `2026-01-12` to `2026-01-18`.

There are 90 whole days since gestational start, so the service output is `12+6`, first trimester, 192 days to EDD, and estimated time since conception `10+4`. The timeline puts the second-trimester start at `2026-04-09` and the third at `2026-07-16`. These are calendar assumptions only.

Enter transfer date `2026-01-20`, embryo age day 5 and `As of` `2026-02-01`. Under the implemented day-5 rule, the service returns EDD `2026-10-08`, gestational start `2026-01-01`, estimated conception `2026-01-15`, and additionally shows the transfer date in the timeline.

For that reference date, the service returns `4+3`, first trimester, 249 days to EDD and estimated time since conception `2+3`. Second- and third-trimester starts are `2026-04-09` and `2026-07-16`. A cycle length does not change this IVF calculation because IVF mode does not use it.

The service labels 0+0 through 13+6 as the first trimester, 14+0 through 27+6 as the second, and 28+0 onward as the third. It also calculates days to or since EDD and marks EDD as `40+0`. Those boundaries structure the timeline; they do not describe individual development.

The module calculates no ultrasound biometry, risks, symptoms, tests, hCG or implantation timing, fertility probability, appointments or sex prediction. It provides no diagnosis and does not assess health or pregnancy progression. A calculated EDD is not a guaranteed birth date, and calendar arithmetic is not ultrasound measurement or medical dating.

With LMP, cycle, ovulation, conception and the window remain estimates of the entered model; irregular cycles can limit them. The calculator is not a reliable contraceptive method and cannot confirm or rule out pregnancy.