Enter the first day of your last period to find your current gestational age, trimester, and due date.

How to Use the Pregnancy Calculator

Enter the first day of your last period, and the calculator returns your current week and day of pregnancy, your trimester, and your estimated due date — no other input needed.

Here's the part that confuses almost everyone the first time they hear it: pregnancy is dated from the first day of your last period, not from conception. Conception happens roughly two weeks after that, around ovulation — which means by the time a pregnancy is "4 weeks along," conception itself is only about two weeks old. It isn't a rounding convention or a quirk of this calculator specifically; it's how obstetric dating has worked for well over a century, mostly because the last period is a known date and conception, for most people, isn't.

Days since LMP = today − LMP
Weeks pregnant = ⌊days ÷ 7⌋, remainder = days mod 7
Due date        = LMP + 280 days

Trimesters follow a fixed convention on top of that same LMP-based count: weeks 1 through 13 are the first trimester, 14 through 26 the second, and 27 through 40 the third. Those boundaries aren't biologically sharp lines — a lot changes gradually rather than on a switch flipped at week 14 — but they're the standard reference point doctors, apps, and this calculator all use, so the label is at least consistent across sources. A cycle that isn't a tidy 28 days shifts this math the same way it shifts every other calculator on this site — someone who ovulates late is earlier in their actual pregnancy than the naive week count suggests, even though the calendar arithmetic on this page looks identical either way.

The due date shown here is the same Naegele's-rule estimate the dedicated Due Date Calculator produces from the same last-period date — this page just adds the current-week framing on top of it.

These are estimates built on average timing, not a clinical assessment. Actual gestational age is best confirmed by a doctor, typically with an early ultrasound, especially if your cycles are irregular or you're unsure of your last period's start date. Use this calculator for a general sense of where you are, not as a substitute for prenatal care.