How this instrument works
Gestational age is the clinical clock obstetrics runs on, and it starts counting from the first day of the last menstrual period rather than from conception — a deliberate convention, not an approximation of biology. A pregnancy is described in completed weeks plus a remainder of days, written as, say, 14+2, and that pair of numbers is what this instrument produces from a single day count. It answers a different question than a due-date calculator does: not 'when will the baby arrive,' but 'where do things stand right now, given the days that have already passed.'
The arithmetic is a floor division. Take the days elapsed, divide by seven, and keep only the whole number of weeks — the fractional remainder, converted back into days, is what gets reported alongside it. A count of 100 days is not a tidy multiple of seven, so it resolves to 14 completed weeks with 2 days left over, not 14.3 weeks; obstetric charts never use fractional weeks, only the weeks-plus-days pair. This is exactly the notation printed on an ultrasound report or read out at a checkup, so the tool exists to reconstruct it from a raw day count whenever it's needed.
One gap is worth naming plainly: because counting starts at the last period rather than at ovulation, gestational age runs roughly two weeks ahead of the embryo's actual age. A cycle that ovulates near day 14 means the embryo itself is only as old as gestational age minus about fourteen days — a fact that surprises plenty of expectant parents comparing this figure with development charts written in embryonic terms. And because the whole calculation depends on a remembered start date, it's only as trustworthy as that date; for cycles that are irregular, or a start date recalled loosely, an early ultrasound measurement becomes the more reliable anchor.
- Count the days elapsed since day 1 of your last menstrual period (the first day of bleeding), or pull the figure from a tracking app.
- Enter that total into Days since LMP.
- Read Gestational age (weeks) and Plus days together — that pair is the standard clinical notation.
- Re-run the figure as days pass; the week count only advances every seventh day, so the day count is the part that moves daily.
Worked example — 100 days since the last period
100 days have passed since day 1 of the last period. Divide by 7: 100 ÷ 7 = 14.28..., and only the whole part counts — 14 completed weeks. Multiply that back out, 14 × 7 = 98, and subtract from the original count: 100 − 98 = 2. The result is 14 weeks, 2 days, written clinically as 14+2.
A cleaner count shows the same logic without a remainder: 84 days is exactly 84 ÷ 7 = 12, so the answer lands on 12+0. Run the same steps on 200 days — 200 ÷ 7 = 28 remainder 4, since 28 × 7 = 196 and 200 − 196 = 4 — and the figure reads as 28 weeks, 4 days, the conventional marker for the start of the third trimester.
Questions
Why does gestational age start counting from my last period instead of conception?
Because the first day of the last period is a fixed, observable date, while the moment of conception almost never is. Clinicians standardized on LMP-based counting decades ago, and the American College of Obstetricians and Gynecologists still frames due-date and gestational-age estimates around it, refined by ultrasound when the two disagree. The convention trades a little biological precision for a start date nearly everyone can name.
What does a result like '14 weeks, 2 days' or '14+2' mean?
It's the same pair of numbers this calculator outputs directly: the whole weeks completed, plus however many extra days sit on top before the next week turns over. A chart, an app, or a sonographer will all write it the same way — 14 weeks and 2 days into the pregnancy, no more, no less.
Is this the same as a due-date calculator?
No. A due-date tool projects forward from a start date to a predicted delivery day, roughly 280 days later. This one does the opposite kind of work: given a day count already in hand, it reports where that count sits today in weeks-and-days form. Neither replaces the other — they answer different questions from the same starting line.
Why does the number sometimes surprise people comparing it to embryo development charts?
Because those charts are often written in embryonic or fetal age, which runs about two weeks behind gestational age. Ovulation typically happens close to day 14 of a cycle, so an embryo is roughly that many days younger than the gestational-age figure suggests. Both numbers are correct — they're measuring from different starting lines.
What if I don't remember my last period precisely, or my cycles are irregular?
Then the output is only as good as the date fed into it. A day count built on a guessed or shifting start date can be off by a week or more, which is exactly the scenario where a first-trimester ultrasound measurement — not a calendar — becomes the more dependable anchor for dating a pregnancy.
Does the week number change every single day?
No — only the day count does that. Because weeks are counted with a floor operation, the week figure holds steady for a full seven-day stretch and then steps up by one all at once; days 91 through 97 are all '13 weeks, x days,' and day 98 is the first day that reads as 14 weeks.
References
- ACOG Committee Opinion No. 700 — Methods for Estimating the Due Date
- StatPearls — Pregnancy Dating (NCBI Bookshelf)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.