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Pregnancy week calculator

Find your current pregnancy week and trimester from your last period.

Related tools

All Pregnancy & baby tools →
Pregnancy CalendarA week-by-week pregnancy timeline with dates and trimesters, with your current week highlighted.Pregnancy Calorie CalculatorDaily calorie needs during pregnancy — TDEE plus the trimester allowance (+340 in T2, +450 in T3).Pregnancy Weight Gain CalculatorRecommended pregnancy weight gain by pre-pregnancy BMI (IOM), with the expected gain at your current week.Due date calculatorEstimate your baby's due date (Naegele's rule).Next period date calculatorEstimate the dates of your next three periods from your last period and cycle length.Ovulation calculatorFind your fertile window from your last period.Baby Growth Percentile CalculatorWeight, length and head circumference percentiles from the WHO Child Growth Standards, 0-24 months.Baby Milk Intake CalculatorBottle volumes per feed and per day by your baby's weight and age, with the feeding interval.

Need Current week, Trimester? The Pregnancy week calculator derives it from First day of last period in one step. For instance, with First day of last period = 1985-03-20 it returns Current week = — and Trimester = none.

How to use it

  1. Enter your values: First day of last period.
  2. Read the result instantly: Current week, Trimester.

Frequently asked questions

What does the Pregnancy week calculator actually compute?

It takes First day of last period and derives Current week and Trimester from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

A single value: First day of last period. Nothing else is required — no account, no file upload.

Can you show a worked example?

With First day of last period = 1985-03-20, the calculator returns Current week = — and Trimester = none. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

When would I actually use this?

Placing a pregnancy on the calendar: the estimated due date, the current week, and when the usual appointments and screenings fall.

What is the most common mistake?

Reading the due date as a deadline. It is a median: fewer than one birth in twenty happens on it, and a normal term spans several weeks around it.

How accurate is it, and what are the limits?

For general guidance only — not medical advice.

What is the difference between the Pregnancy week calculator and the Pregnancy Calendar?

This one returns Current week and Trimester; the Pregnancy Calendar returns Result. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Pregnancy Calorie Calculator is the closest one after this: Daily calorie needs during pregnancy — TDEE plus the trimester allowance (+340 in T2, +450 in T3).

What else is worth having open alongside it?

Pregnancy Weight Gain Calculator and Due date calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from, and how current are they?

Dating follows Naegele's rule (280 days from the last period) or the transfer date for assisted conception. An early ultrasound is more accurate than any calculation and takes precedence over it.

Further reading

All guides →
ExplainerPregnancy: Weeks to MonthsPregnancy is counted in weeks, but months aren't exactly four weeks. Here's why it's 40 weeks, how weeks map to months and trimesters, and a quick reference.How-toHow to Calculate Your Due DateNaegele's rule adds 280 days to your last period — or subtract three months and add seven days. Here's how it works and why it's only an estimate.ExplainerOvulation and Your Fertile WindowOvulation is usually 14 days before your next period, and the fertile window spans about six days. Here's how it works and how to estimate yours.GuidePregnancy Week by Week: The Calendar and What Each Stage MeansPregnancy is counted in 40 weeks from your last period, not from conception — so you are already two weeks pregnant at conception. Here is the full calendar, the trimester boundaries and what term actually means.GuideHow Much Weight to Gain in Pregnancy: The Guidelines by Starting BMIThe recommended range depends entirely on your BMI before pregnancy: 28 to 40 lb if you started underweight, 11 to 20 lb if you started with obesity. Here is the full table and the weekly rate behind it.ExplainerWhat Is a Normal Menstrual Cycle Length? The Numbers, by Life StageFIGO puts a normal cycle at 24 to 38 days; ACOG's older range is 21 to 35. Here is why two respected bodies give different numbers, how much variation is normal, and how cycles change after menarche and before menopause.