Pregnancy is dated from your last period, not from conception, which is why you’re already “4 weeks along” right around the time a test first turns positive.
Free pregnancy tool
Pregnancy Calculator
Your current week and trimester, due date, and the key milestone dates ahead, from your last period.
Runs entirely in your browser. Nothing you enter is sent anywhere. Only need the due date itself, or working from a known conception or IVF transfer date instead? Try the Due Date Calculator.
Why pregnancy is dated from your last period, not conception
It looks backwards, but "gestational age" is counted from the first day of your last menstrual period, roughly two weeks before conception actually happens, because that date is known precisely while the exact conception date usually isn't. This means you're already considered "4 weeks pregnant" around the time a pregnancy test would first turn positive. The trimester boundaries and milestones below use the same widely-cited convention: first trimester through week 13, second trimester weeks 14 to 27, third trimester from week 28. Viability and full-term dates are population averages from clinical guidance, not a guarantee for any individual pregnancy, your own care provider's dating (especially after a dating ultrasound) is the one that actually matters for your care.
Pregnancy calculator FAQ
Why am I already "4 weeks pregnant" right after a positive test?
Because gestational age is counted from the first day of your last period, roughly two weeks before conception actually happens, not from conception itself. A pregnancy test typically turns positive around 4 weeks gestational age, which is only about 2 weeks after conception.
What is the difference between gestational age and fetal age?
Gestational age (used here and by most care providers) counts from the last period. Fetal or "conceptional" age counts from conception itself, roughly 2 weeks less. If a number from another source looks 2 weeks off from this calculator, that is almost always the reason.
When does viability start?
Around 24 weeks gestational age is the commonly cited threshold where survival outside the womb becomes possible with intensive medical care, though outcomes improve substantially with every additional week after that. It is a population statistic, not a guarantee for any individual case.
What counts as "full term"?
37 weeks through 41 weeks gestational age is considered full term (with early term, full term, late term and post-term as finer sub-bands used clinically). Before 37 weeks is preterm.
Is my data private?
Yes. Every calculation runs in your browser. Nothing you enter is sent to a server, logged, or stored.
Free pregnancy calculator by ANUPRESS
Gestational age versus fetal age
Gestational age, the number this calculator and most care providers use, counts from the first day of your last period. Fetal or “conceptional” age counts from conception itself, roughly two weeks less. If a number from an app, a friend, or another source looks two weeks off from what’s shown here, that difference in convention is almost always the reason, not an error in either calculation.
The milestones, and what they actually mean
End of first trimester (week 13), the viability milestone (around week 24), and full term (starting week 37) are population averages drawn from clinical guidance, not guarantees for any individual pregnancy. Viability in particular describes when survival outside the womb becomes possible with intensive care, with outcomes improving substantially each additional week after that, it is not a single hard line.
When your own dating differs from this calculator
A dating ultrasound, usually done in the first trimester, is more accurate than a last-period estimate alone and is what your actual care provider will use going forward once it’s available. If your provider’s dates differ from this calculator’s, theirs is the one that matters for your care, this tool is for a quick estimate, not a substitute for clinical dating.