Gestational Week
Overview
Obstetric dating counts from the first day of the last menstrual period, not from conception, so gestational age is the floor of elapsed days divided by seven. A pregnancy 100 days past the last period stands at 14 weeks and 2 days, written 14+2 in clinical notation. The convention adds roughly two weeks to true embryonic age because ovulation typically occurs around day 14, which is why a person is nominally two weeks pregnant at the moment of conception. Trimesters follow from the same clock: the first runs to the end of week 13, the second through week 27 and the third to delivery, with term defined as 37 to 42 weeks. The method is used because the last period is a date a patient can usually recall while ovulation is not, though first-trimester ultrasound biometry supersedes it when the two estimates diverge materially.
Variables
| Symbol | Name | Unit | Description |
|---|---|---|---|
| $LMP$ | Last menstrual period | date | First day of the last menstrual period, the conventional day zero. |
| $today$ | Reference date | date | The date at which gestational age is being assessed. |
| $Week$ | Gestational week | weeks | Completed weeks since the LMP, reported as weeks plus days. |
Dated From a Period, Not From Conception
Clinical gestational age counts from the first day of the last menstrual period rather than from conception:
$$Week = \left\lfloor\frac{today - LMP}{7}\right\rfloor$$
The floor function returns completed weeks; the remainder gives the days, which is why obstetric records read "24 weeks and 3 days" rather than a decimal.
The Two-Week Offset
Ovulation occurs roughly two weeks after the LMP in a 28-day cycle, so gestational age exceeds embryonic age by about a fortnight throughout. A pregnancy described as six weeks gestational involves an embryo of about four weeks. Full term is reached at 40 weeks gestational, or about 38 weeks from conception.
Trimesters and Viability
| Milestone | Gestational week |
|---|---|
| First trimester ends | 13 |
| Second trimester ends | 27 |
| Threshold of viability | 24 |
| Full term | 39–40 |
| Post-term | 42 |
Why an Imperfect Landmark Is Used
The LMP is observable and usually recallable; conception is neither. Dating from an event the patient can report makes the measure available at the first consultation, before any imaging, at the cost of a systematic offset that is consistent enough to be built into every reference table.
Derivation & History
The convention descends from Naegele's rule, published in the early nineteenth century, which set the estimated due date at the LMP plus one year minus three months plus seven days — equivalent to 280 days, or exactly 40 weeks. Fixing 280 days as the standard gestation implied a 40-week scale dated from the LMP, and weekly reporting followed because fetal development benchmarks and screening windows are naturally spaced in weeks rather than in days or months.
Worked Examples
LMP 1 January 2026, assessed 20 June 2026
- Elapsed days: 170
- Completed weeks: ⌊170 ÷ 7⌋ = 24
- Remainder: 170 − 168 = 2 days
Result: 24 weeks and 2 days
Edge Cases & Limitations
Irregular cycles break the assumption: a cycle materially longer or shorter than 28 days shifts ovulation away from day 14, so LMP dating misstates gestational age by the difference.
Ultrasound supersedes the LMP: a first-trimester crown-rump measurement dates the pregnancy more accurately, and clinical practice redates when the two disagree beyond a set tolerance.
An uncertain or unknown LMP: recall error is common, and where the date is not known the expression cannot be applied at all.
The result is an estimate: fewer than one in twenty births occurs on the calculated due date, and a term delivery spans weeks 37 to 42.
Real-World Applications
Gestational week determines the timing of every routine antenatal step — the nuchal translucency scan near week 12, the anomaly scan near week 20, glucose screening near week 26 — and it establishes the viability threshold that shapes neonatal care decisions. Employers and insurers reckon maternity leave entitlements from the same estimated due date, and neonatologists classify a newborn as preterm, term or post-term on this scale.