Gestational age starts at the last menstrual period, while fetal age starts at conception. See why the figures differ by about two weeks and what Naegele’s rule assumes.
There is a moment in most pregnancies where two numbers refuse to agree. The maternity notes say one thing, an app says something two weeks different, and a relative offers a third figure with complete confidence.
All three can be right. There are two distinct clocks in use, they differ by exactly fourteen days, and almost nobody explains which is which.
The two clocks
Gestational age is counted from the first day of your last menstrual period. It is what clinicians use, what appears in your notes, what scan reports state, and what every piece of official guidance means. Full term is forty weeks.
Fetal age, sometimes called conceptional or embryonic age, is counted from estimated conception. It reflects actual developmental time. Full term is thirty-eight weeks.
| Gestational age | Fetal age | |
|---|---|---|
| Counted from | First day of last period | Estimated conception |
| Full term at | 40 weeks | 38 weeks |
| Used by | Clinicians, notes, scans, guidance | Embryology texts, some apps |
| Advantage | Based on an observable, recallable date | Reflects true developmental age |
| Drawback | Includes two weeks before pregnancy began | Requires knowing the conception date |
The gap is not an approximation. It is a fixed fourteen days, because gestational dating assumes ovulation on day fourteen of a twenty-eight day cycle.
Why count from before the pregnancy began
It looks illogical to start the clock two weeks before conception. The reason is entirely practical: the last menstrual period is a date people can actually observe and remember. Conception is not.
Ovulation produces no reliable outward sign. Fertilisation happens invisibly. Implantation happens days later, also invisibly. Sperm can survive several days in the reproductive tract, so intercourse on a known date does not fix conception to that date.
The onset of a period, by contrast, is unmistakable, dateable, and frequently recorded. Clinical dating uses the one landmark that exists.
The cost is a permanent two-week offset between the number in your notes and the biological age of the pregnancy. The benefit is that the number can be established at the first appointment from a question anyone can answer.
Naegele’s rule and its single assumption
The formula behind almost every due date given anywhere in the world dates from the early nineteenth century and the German obstetrician Franz Karl Naegele: add 280 days to the first day of the last menstrual period.
It is usually taught as a mental shortcut, add one year, subtract three months, add seven days, which gives the same answer give or take the lengths of the months involved. Our Due Date Calculator does it on real calendar dates instead, so leap years and month lengths are handled exactly.
The 280 days splits cleanly:
280 days = 14 days from period to ovulation + 266 days of gestation
The second number is biological. The first is an assumption about your cycle, and it is the only part of the formula that can be wrong for a specific person.
Cycle length changes the answer by up to a fortnight
Cycle length varies substantially between people. What stays comparatively constant is the luteal phase, the interval from ovulation to the next period, which is typically around fourteen days regardless of total cycle length.
That means a longer cycle implies a later ovulation, not a longer wait afterwards. Which gives a clean correction: for every day your cycle exceeds 28, add a day to the due date.
| Cycle length | Estimated ovulation | Correction |
|---|---|---|
| 21 days | Day 7 | 7 days earlier |
| 25 days | Day 11 | 3 days earlier |
| 28 days | Day 14 | None, the textbook case |
| 32 days | Day 18 | 4 days later |
| 35 days | Day 21 | 7 days later |
A fortnight separates the 21-day and 35-day estimates from the same period date. This is the single largest source of disagreement between due-date calculators, and most simple ones do not apply the correction at all.
The correction has limits. It assumes regular cycles and a roughly fourteen-day luteal phase. If cycles are irregular, the period date itself becomes unreliable, which is exactly why an early scan is offered.
Why the scan overrides the arithmetic
A first-trimester ultrasound measures the fetus directly, typically via crown-rump length, and early development is remarkably uniform across pregnancies. That makes early measurement a more accurate dating method than menstrual history.
Standard clinical practice is that if the scan date differs from the menstrual date by more than a few days, the scan generally takes precedence. The threshold and the exact rule are set by professional guidance and vary somewhat, and your care provider works to the current version.
Accuracy degrades as pregnancy advances, because growth rates diverge between individuals. A dating scan at eight weeks is considerably more precise than a growth estimate at thirty weeks. This is why dating is established early and then held fixed, the due date does not get revised later on the basis of size.
Menstrual dating works well when
- Cycles are regular and the period date is known confidently
- Cycle length is known, so the correction can be applied
- No hormonal contraception was in use in the preceding months
- You need an estimate before any scan is available
Scan dating is needed when
- Cycles are irregular or the period date is uncertain
- Conception occurred shortly after stopping hormonal contraception
- There was bleeding that may have been mistaken for a period
- The menstrual estimate and the measured size disagree substantially
- The pregnancy is a multiple
What “term” means now
“Term” used to cover 37 to 42 weeks as a single safe window. That changed, because outcomes across that five-week range turned out not to be uniform.
| Category | Gestational age |
|---|---|
| Preterm | Before 37 weeks |
| Early term | 37w0d to 38w6d |
| Full term | 39w0d to 40w6d |
| Late term | 41w0d to 41w6d |
| Post-term | 42w0d onward |
The distinction between early term and full term is the reason elective procedures are generally not scheduled before 39 weeks without a clinical indication. A birth at 37 weeks is term, but it is not the same as a birth at 39.
Note the notation: weeks and days, written as 38w4d. That format is standard in clinical records and it is what our calculator reports, because “eight and a half months” is not a unit anyone can act on.
The due date is a midpoint, not a prediction
Only a small minority of births occur on the estimated date itself. The great majority arrive somewhere in a window of roughly two weeks either side.
That spread is normal biology, not error in the calculation. Which has two practical consequences.
First, arrangements pinned to the exact due date are pinned to the least likely single outcome. A partner’s leave beginning that morning, a relative flying in for that week, a car booked for that day, all of these are betting on the one day the baby is least likely to arrive relative to the whole window. Planning for a range from about 37 weeks is the version that survives.
Second, the disagreement between due-date calculators matters less than it appears. A four-day difference from a cycle-length correction sits well inside the natural spread. The correction is worth applying because it centres the estimate better, not because it narrows the window.
Reading week-by-week content without confusion
Most of the confusion in pregnancy apps and books comes from mixing the two clocks. Some sources describe development in fetal age, some in gestational age, and many do not say which.
The tell is usually the total. If a source says pregnancy lasts 38 weeks, it is using fetal age. If it says 40, it is gestational. If a “week 8” description mentions features you were told to expect at week 10, you are looking at fetal age.
Whenever a number appears without a stated clock, compare it against your notes. Your notes are gestational, always. Two weeks’ difference is the signature of the other clock, not an error.
The practical summary
- Use gestational age for everything. It is what your notes, your appointments and all official guidance use.
- Know your cycle length. It is the one input that meaningfully changes the arithmetic, and most calculators ignore it.
- Expect the scan to win. Early measurement beats menstrual history, and dating is fixed early rather than revised later.
- Treat the due date as a window. Roughly two weeks either side is entirely normal.
- Watch for the two-week offset when a source disagrees with your notes.
Our Due Date Calculator applies Naegele’s rule with the cycle-length correction, reports gestational age in weeks and days, and marks each term boundary as a date. It is arithmetic, not care, your midwife or doctor has information no calculator can have, and their date supersedes any figure produced here.
Once the baby arrives, age is tracked in weeks and then months for the first two years, because annual units are far too coarse for that period. The Age Calculator reports both.