Pregnancy · Dating
How many weeks pregnant am I?
Calculate your current pregnancy week, trimester and percent-complete from your last period (LMP), your due date, or your IVF transfer date.
Last reviewed 24 May 2026
Current gestational age
How many weeks pregnant am I?
What you know
Pick a method and enter the date to see your current pregnancy week.
How to use this calculator
Pick the reference you know — your last menstrual period (LMP), your due date (EDD), or your IVF transfer date — and enter it. The calculator returns your current gestational age in weeks-and-days, which trimester you’re in, your estimated due date, days remaining and percent complete.
The math
- From LMP: gestational age = today − LMP. EDD = LMP + 280 days (Naegele).
- From EDD: gestational age = 280 − (EDD − today). Equivalent to working back to an LMP and forward to today.
- From IVF transfer: we recover a notional LMP = transfer − (embryo age + 14 days), then count forward as above. A Day-5 blastocyst transferred today is gestationally about 2 weeks 5 days old.
Trimester boundaries used here
- First trimester: weeks 0 through 13.
- Second trimester: weeks 14 through 27.
- Third trimester: weeks 28 onward.
Limitations
- LMP-based dating assumes regular cycles with mid-cycle ovulation; PCOS, irregular cycles or shift-work disruption all introduce error.
- A first-trimester ultrasound (8–13 weeks, CRL measurement) is the most accurate dating tool and may override an LMP-based estimate that’s off by more than 5 days.
- Twins / multiples don’t change gestational-age math but tend to deliver before the standard 40-week EDD.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol 2017;129:e150-4.
- Royal College of Obstetricians and Gynaecologists. Pregnancy dating after assisted conception.
Frequently asked questions
What's the difference between gestational age and fetal age?
Gestational age (the number this calculator returns) is measured from the first day of your last period and is the standard obstetric clock — it's what your doctor uses. Fetal age is measured from the day of conception, which is roughly 2 weeks behind gestational age. When someone says 'I'm 12 weeks pregnant', they mean 12 weeks gestational age — the actual embryo is about 10 weeks old.
Why does pregnancy use a 40-week count when conception happens later?
Naegele's rule (1812) chose LMP as the reference point because most women can identify the first day of their period more reliably than the day of ovulation. The 280-day / 40-week convention has stuck even though it overstates the actual fetal age by ~2 weeks — every obstetric resource and ultrasound machine still uses it.
When do the trimesters start and end?
The standard convention used here is: first trimester = weeks 0–13, second trimester = weeks 14–27, third trimester = weeks 28 onward. Some older sources end the first trimester at week 12 — the difference reflects whether you count by completed weeks or weeks plus days. Either way the rough mid-pregnancy mark is around week 20.
My LMP-based week doesn't match my ultrasound. Which is right?
The ultrasound is more accurate, especially before 13 weeks when the crown-rump-length measurement is reliable. ACOG recommends overriding the LMP date if it differs from a first-trimester ultrasound by more than 5 days. Once you're past 13 weeks the ultrasound dating is less precise and the original LMP-based date usually stands.