# Due Date

Estimate a due date from a last menstrual period, a known conception or transfer date, or an ultrasound measurement — then see what the estimate actually rests on. Adjusts Naegele's rule for cycle length, applies the ACOG thresholds for when an ultrasound should replace period-based dating, and reports gestational age, trimester boundaries, and the term windows.

**Not for:** Not medical advice and not a substitute for clinical dating by a qualified clinician. This is arithmetic on the dates you supply; it cannot see an ultrasound, a chart, or a person, and it does not know about multiples, irregular cycles beyond the length you enter, or any complication. Estimated dates are estimates — a small minority of births happen on them.

## The three things this gets right

1. Gestational age is counted from the first day of the LAST MENSTRUAL PERIOD, not from conception. At six weeks pregnant, conception was about four weeks ago.
2. Naegele's rule assumes a 28-day cycle with ovulation on day 14. For a 35-day cycle the unadjusted estimate is a week out. This adjusts for it.
3. Whether an ultrasound should REPLACE the period-based date depends on the discrepancy AND on gestational age at the scan. This applies the ACOG Committee Opinion 700 thresholds rather than guessing at them.

## Methods

| Method | Arithmetic |
| --- | --- |
| `lmp` | last period + 280 days, plus (cycleLength - 28) |
| `conception` | conception + 266 days |
| `ivf-day3` | transfer + 263 days |
| `ivf-day5` | transfer + 261 days |
| `ultrasound` | scan date + (280 - measured gestational age) |

## Surfaces

- HTML: `https://due-date.gumballtools.com`
- Markdown: same URLs with `Accept: text/markdown` or `?format=md`
- JSON API: `https://due-date.gumballtools.com/api/v1/estimate`
- OpenAPI: `https://due-date.gumballtools.com/.well-known/openapi.json`
- MCP: `https://due-date.gumballtools.com/api/mcp`

Dates must be `YYYY-MM-DD`. Free-form dates are refused, not guessed.

## Worked examples

- [Due date from the last menstrual period](https://due-date.gumballtools.com/dating/due-date-from-last-period) — Naegele's rule: 280 days from the first day of the last period. The same thing as the paper wheel, with the assumption it makes written down.
- [Due date from a known conception date](https://due-date.gumballtools.com/dating/due-date-from-conception-date) — Conception plus 266 days, not 280. The fourteen-day difference is the gap between the period and ovulation, and it is where most manual errors come from.
- [IVF due date after a day-3 transfer](https://due-date.gumballtools.com/dating/ivf-due-date-day-3-transfer) — Transfer date plus 263 days. The embryo was already three days old at transfer, so the offset is shorter than from conception.
- [IVF due date after a day-5 blastocyst transfer](https://due-date.gumballtools.com/dating/ivf-due-date-day-5-transfer) — Transfer date plus 261 days. IVF dating is the most precise there is, and a later scan should not override it.
- [Due date with a long (35-day) cycle](https://due-date.gumballtools.com/dating/due-date-with-a-35-day-cycle) — A week later than the unadjusted estimate, because ovulation is a week later. Most calculators skip this adjustment entirely.
- [Due date with a short (24-day) cycle](https://due-date.gumballtools.com/dating/due-date-with-a-short-cycle) — The adjustment runs both ways. A shorter cycle means earlier ovulation and an earlier estimate.
- [Due date from an ultrasound measurement](https://due-date.gumballtools.com/dating/due-date-from-an-ultrasound) — Worked backwards from the gestational age the scan reported. With no period date supplied there is nothing to cross-check it against.
- [When an ultrasound should change the due date](https://due-date.gumballtools.com/dating/when-an-ultrasound-changes-the-due-date) — Six days apart at an eight-week scan. ACOG puts the threshold at five days that early, so the ultrasound date replaces the period-based one.
- [When an ultrasound should not change the due date](https://due-date.gumballtools.com/dating/when-an-ultrasound-does-not-change-the-due-date) — The scan agrees with the period date, so nothing moves. A scan that confirms the dating is not a scan that redates it.
- [Why the same disagreement stops mattering later](https://due-date.gumballtools.com/dating/redating-later-in-pregnancy) — Six days apart again — but at twenty weeks the threshold is ten days, so the period date stands. Ultrasound dating loses precision as pregnancy advances.
- [How many weeks pregnant am I?](https://due-date.gumballtools.com/dating/how-many-weeks-pregnant-am-i) — Counted from the last period, which is the convention every clinician uses — and about two weeks more than time since conception.
- [When does the second trimester start?](https://due-date.gumballtools.com/dating/when-does-the-second-trimester-start) — At 14w0d by the common convention. The trimester boundaries are conventions, not biological events.
- [Is 37 weeks full term?](https://due-date.gumballtools.com/dating/is-37-weeks-full-term) — No. ACOG stopped calling it that in 2013. From 37w0d to 38w6d is EARLY term; full term begins at 39w0d.
- [Past the due date at 41 weeks](https://due-date.gumballtools.com/dating/past-the-due-date-at-41-weeks) — The due date came and went, which is the ordinary case rather than the exception. Here is what the classification actually says.