Gestational age counts pregnancy in weeks from the first day of your last menstrual period — a full term is 40 weeks, with 37 to 42 considered term. This free calculator finds your current week and trimester; a first-trimester ultrasound, accurate to about ±5–7 days, officially replaces the period-based date when they disagree.
“How far along am I?” is the first question of every pregnancy — and the answer has its own counting system. Gestational age is measured in weeks from the first day of your last menstrual period, not from conception, and it anchors everything that follows: scan schedules, screening windows, and the due date itself. This gestational age calculator finds your current week from your last period date.
Get your week below, then read on: how the dating works, weeks-to-months conversion, the trimester map, and why an early ultrasound can legitimately change your date.
Gestational Age
Your current pregnancy week from the last period.
How far along am I?
Gestational age = the time since the first day of your last menstrual period (LMP). If your last period started 10 weeks and 3 days ago, you are “10 weeks 3 days” — written 10+3. A full-term pregnancy runs to 40 weeks by this clock, with birth anywhere from 37 to 42 weeks considered term. The quirk everyone trips over: the first two weeks of “pregnancy” happen before conception, because the clock starts at the period, not at fertilization.
How many weeks pregnant am I — by milestone
| Weeks | Months (approx.) | Trimester | Key events |
|---|---|---|---|
| 4 | 1 | First | Missed period; positive test possible |
| 8 | 2 | First | First scan window; heartbeat visible |
| 12–13 | 3 | First | First-trimester screening; dating scan |
| 14–27 | 4–6 | Second | Anatomy scan (~20 weeks); movements felt |
| 28–36 | 7–8 | Third | Growth scans; glucose screening done by now |
| 37–40+ | 9 | Third | Term; delivery expected |
The “weeks to months” conversion is always approximate — months vary in length, which is why obstetrics counts exclusively in weeks.
LMP vs ultrasound: which date wins?
LMP dating assumes a 28-day cycle with ovulation on day 14 — a fine average and a poor fit for long, short or irregular cycles. That is why the first-trimester ultrasound, which measures the embryo’s crown-rump length, is the gold standard: accurate to about ±5–7 days. Standard practice: if the early scan differs from your LMP date by more than about 5–7 days, the scan date officially replaces it. A calculator gets you started; the dating scan settles it.
Gestational age vs fetal age
Fetal (conceptional) age = gestational age − 2 weeks, because conception happens around two weeks after the period the count starts from. At “12 weeks pregnant” the embryo has existed for about 10. Medicine, apps and this calculator all speak gestational age; embryology textbooks speak fetal age — knowing the offset prevents endless confusion. To work from the other direction — finding the conception date — use our conception calculator, and for the delivery end, the due date calculator.
When dating matters medically
Accurate dating is not pedantry — screening tests are only valid in specific windows (first-trimester combined screening at 11–14 weeks, anatomy scan around 18–22), decisions about early or late delivery hang on the true week, and growth assessment is meaningless against the wrong baseline. If your cycles are irregular or your LMP date is uncertain, say so at your first visit — it changes how your care team dates the pregnancy. Acibadem’s obstetrics specialists combine dating scans with full antenatal care.
This tool is informational. Official pregnancy dating is set by your obstetrician, usually from a first-trimester ultrasound. Always follow your own care team’s dates and schedule.
Frequently asked questions
How do I calculate how far along I am?
Count the weeks from the first day of your last menstrual period to today — that is your gestational age. The calculator above does it precisely and shows the corresponding trimester.
Why does pregnancy count from the last period and not conception?
Because the period date is usually known while the conception date is usually not. The convention adds about two weeks before conception to the count — universal across medicine.
How many weeks is a pregnancy?
40 weeks from the last period, with 37–42 weeks considered term. Only about 4–5% of babies arrive on the due date itself.
Can my due date change after an ultrasound?
Yes — if a first-trimester scan disagrees with your period-based date by more than about 5–7 days, the scan becomes the official date. Later scans are less accurate for dating and rarely change it.
What if my cycles are irregular?
LMP math assumes ovulation on day 14, so irregular cycles make it unreliable — the dating ultrasound becomes essential. Tell your obstetrician your cycle history at the first visit.
Is gestational age the same as fetal age?
No — fetal age runs about two weeks behind gestational age, because conception occurs roughly two weeks after the last period began.
Numbers are a starting point — not a diagnosis
If your results raise questions, Acibadem specialists can review your situation properly: share your case and receive a free, no-obligation medical opinion from the relevant department.