Pregnancy Week Calculator
Your current week, trimester, and weeks to go.
Pregnancy Week Calculator
See how many weeks and days pregnant you are, the trimester, fetal age, and what comes next.
Gestational age is counted from the first day of your last period.
Change this to see a past or future week.
Week 14+0 days
Second trimester (weeks 13–27)
- Fetal age
- 12w 0d
- Estimated due date
- 2027-04-30
- Weeks to go
- 26
- Estimated conception
- 2026-08-07
Next touchpoint: week 20 � Anomaly scan window (18–22 weeks).
This is an estimate based on calendar dates, not a medical diagnosis or a dating scan. Only about 4% of babies arrive on their estimated due date. Speak to your midwife, GP, or obstetrician about dating and any symptoms that worry you.
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What This Calculator Does
The Pregnancy Week Calculator tells you how many weeks and days pregnant you are on any date, counted from the first day of your last period. It also shows the trimester, fetal age, the estimated due date, and weeks remaining.
How It Works
Obstetric dating counts from the first day of the last menstrual period, not from conception. Gestational age is the number of days since that date divided into weeks and days. Fetal age is about two weeks less, because ovulation happens roughly two weeks into the cycle. With a known conception date, the calculator converts it to the equivalent LMP before counting.
Example
If your last period began on 1 January 2026, then on 11 April 2026 you are 100 days pregnant — 14 weeks and 2 days. Fetal age is 12 weeks 2 days, the estimated due date is 8 October 2026, and about 26 weeks remain.
Formula
Weeks = floor((today − LMP) / 7); Days = (today − LMP) mod 7Fetal age ≈ gestational age − 14 daysApproximate because it assumes ovulation on cycle day 14.
Weeks to go = ceil((due date − today) / 7)Reading the Results
Week and days
Your gestational age in the format used by midwives and scan reports. “Week 14 + 2” means 14 completed weeks and 2 days.
Fetal age
Time since fertilisation, about two weeks less than gestational age. It is useful for reading development information, which often uses fetal age.
Next milestone
The next standard antenatal window or trimester boundary. It is a planning marker, not a substitute for the schedule your own clinician gives you.
Sources and References
ACOG Committee Opinion 700 — Methods for Estimating the Due Date
Gestational age is counted from the LMP
ACOG recommends revising the estimated due date when an early ultrasound differs materially from calendar dating.
NICE NG201 — Antenatal care
Dating scan at 11–14 weeks; anomaly scan at 18–22 weeks
The milestone labels in this tool reflect the usual UK antenatal schedule, not an appointment booking.
References identify the method or data behind this calculator. Where a standard or tariff changes, the current edition applies, and the assumptions stated above take precedence over a remembered figure.
Frequently Asked Questions
How do I calculate how many weeks pregnant I am?
Count the days from the first day of your last period to today, then divide by seven. The whole number is your completed weeks and the remainder is the extra days. This calculator does that and also handles a known conception date.
Why is fetal age two weeks less than gestational age?
Pregnancy is dated from the last period, but fertilisation happens about two weeks later. So at 14 weeks gestational age, the embryo or fetus has been developing for roughly 12 weeks. The two-week gap is a convention, not a discrepancy.
What if my scan gives a different gestational age?
An early ultrasound is more accurate than calendar dating, especially with irregular cycles. Clinicians usually revise the due date when the scan differs materially before 14 weeks, so use the scan-derived date for appointments.
Does this calculator tell me if everything is progressing normally?
No. It only does calendar arithmetic. It cannot assess growth, position, or wellbeing. Any concern about movement, pain, bleeding, or growth should go to your midwife or doctor the same day.