BYTETOOLS

Pregnancy Week Calculator

Work out how many weeks and days pregnant you are, plus your trimester and due date, from your last period, conception date or a known due date.

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What is the Pregnancy Week Calculator?

A pregnancy week calculator converts a dating anchor into gestational age. Gestational age is counted in weeks and days from the first day of your last menstrual period, so it includes the roughly two weeks before conception actually happened.

  • Three dating methods: last period, conception date or a known due date
  • Gestational age in weeks + days, with a progress bar to 40 weeks
  • Cycle-length adjustment so long or short cycles date correctly
  • Trimester, days remaining and approximate months at a glance
  • Milestone table marking passed and upcoming landmarks
  • Fully offline — your dates never leave the browser

How to use the Pregnancy Week Calculator

  1. 1

    Choose whether you are dating from your last period, a conception date or an existing due date.

  2. 2

    Enter that date, and your average cycle length if you picked the last-period method.

  3. 3

    Check the “Today” field, or change it to date a different day.

  4. 4

    Read your gestational age in weeks and days, your trimester and your due date.

  5. 5

    Scroll the milestone table to see which pregnancy landmarks you have passed.

About the Pregnancy Week Calculator

The ByteTools Pregnancy Week Calculator tells you how many weeks and days pregnant you are today, which trimester that falls in, and how long is left until your due date. You can work from the first day of your last menstrual period, from a known conception or IVF-transfer date, or backwards from a due date a clinic has already given you.

Gestational age is the number obstetric care actually runs on, and it is counted from the last period rather than from conception — which is why a "six week" pregnancy is only about four weeks past conception. The tool shows a milestone table with the end of the first trimester, the anatomy-scan window, the start of the third trimester and the full-term window, each marked as passed or upcoming.

Everything runs locally in your browser and no dates are uploaded or stored. This is a date calculation and nothing more — it is not medical advice, it cannot confirm a pregnancy, and it cannot tell you anything about how one is progressing. A first-trimester dating scan is far more accurate than any menstrual-date formula, so always follow the dates your midwife or doctor gives you.

Frequently asked questions

How many weeks pregnant am I?

Count the days from the first day of your last menstrual period to today, then divide by seven — the remainder is the extra days. So 86 days is 12 weeks and 2 days. This calculator does that for you and also handles dating from a conception date or an existing due date.

Why does pregnancy start counting two weeks before conception?

Because the last menstrual period is the one date most people can identify reliably, while the exact day of conception usually is not known. The convention adds about two weeks at the front, which is why gestational age is roughly two weeks ahead of the actual age of the embryo.

How accurate is dating from the last menstrual period?

Less accurate than an ultrasound. It assumes you ovulated on day 14 of a regular cycle, which often is not the case — irregular or long cycles can shift the dates by a week or more in either direction. A first-trimester dating scan measures the embryo directly and is accurate to within about five days, which is why clinics re-date pregnancies when the two disagree.

When does each trimester start and end?

The common convention is first trimester from 0 weeks to 13 weeks 6 days, second from 14 weeks 0 days to 27 weeks 6 days, and third from 28 weeks 0 days until birth. Some clinicians draw the boundaries slightly differently, so do not be surprised by a week's discrepancy.

How is this different from a due date calculator?

A due date calculator answers one question: when is the baby expected. This one answers the everyday question — how far along am I right now — and works in either direction, so you can also feed in a due date you already have and get today's gestational age back.

Is my information kept private?

Yes. Every calculation happens in your browser using JavaScript. No dates are transmitted, logged or stored anywhere.

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