Pregnancy Week Calculator
Find out how many weeks pregnant you are and which trimester you are in.
Adjusts the estimated due date only
Defaults to today; change it to see your progress on any date
How Far Along
Trimester
Estimated Due Date
Pregnancy Progress
How the Pregnancy Week Calculator Works
Pregnancy is measured in gestational age (GA), the number of completed weeks and days since the first day of your last menstrual period (LMP). A full-term pregnancy lasts about 40 weeks, or 280 days, counted from the LMP. This calculator works out exactly how many weeks and days along you are on any chosen date, which trimester you are in, how much of the pregnancy is complete, your estimated due date, and how many days remain.
Trimester Ranges
| Trimester | Gestational Age | Approx. Months |
|---|---|---|
| First Trimester | 0 weeks 0 days – 13 weeks 6 days | Months 1–3 |
| Second Trimester | 14 weeks 0 days – 27 weeks 6 days | Months 4–6 |
| Third Trimester | 28 weeks 0 days – birth | Months 7–9 |
Why Pregnancy Is Counted From Your Last Period
Gestational Age vs. Conception Age
By medical convention, gestational age is counted from the first day of the last menstrual period — roughly two weeks before conception actually occurs. This means that at the moment of conception you are already considered about 2 weeks pregnant. LMP dating is used because the date of the last period is far easier to know reliably than the exact day of ovulation or fertilization. As a result, your "weeks pregnant" will be about 2 weeks more than the time since conception (the embryonic or fetal age).
The Calculations
From LMP: GA (days) = As-of date − LMP date
Due date (LMP): LMP + 280 days + (cycle length − 28)
From due date: GA (days) = 280 − (Due date − As-of date)
Progress: GA ÷ 280 × 100%
The cycle-length adjustment shifts the estimated due date for women whose cycles are shorter or longer than the typical 28 days (a method known as Naegele's rule with adjustment). Standard obstetric week counting itself is always measured from the LMP, regardless of cycle length.
Frequently Asked Questions
How accurate is this estimate?
- • LMP dating assumes a regular cycle and known period date
- • A first-trimester ultrasound is the most accurate dating method
- • Due dates are estimates — only about 1 in 20 babies arrive on the exact date
- • Most births occur within 1–2 weeks of the due date
What does "24 weeks 3 days" mean?
- • You have completed 24 full weeks of pregnancy
- • Plus 3 additional days into your 25th week
- • Clinicians often write this as "24+3"
- • Full term is reached at 37 weeks 0 days
I have irregular cycles — what then?
- • LMP-based dating may be less reliable
- • Enter your average cycle length to refine the due date
- • If you know your due date from a scan, use the "By Due Date" method
- • Ask your provider for ultrasound-based dating
Why count weeks at all?
- • Tracks fetal development milestones week by week
- • Schedules screenings and tests at the right time
- • Helps assess whether a baby is preterm, term, or post-term
- • Lets you follow trimester-specific changes
References
The dating conventions and trimester definitions used in this calculator are based on established obstetric guidelines:
- American College of Obstetricians and Gynecologists (ACOG). "Methods for Estimating the Due Date." Committee Opinion No. 700. ACOG Committee Opinion 700
- National Institute for Health and Care Excellence (NICE). "Antenatal care." NICE guideline NG201. NICE NG201
- World Health Organization (WHO). "WHO recommendations on antenatal care for a positive pregnancy experience." WHO Antenatal Care
Note: This calculator provides estimates based on standard dating rules and should not replace professional medical advice. The most accurate way to date a pregnancy is an ultrasound performed in the first trimester. Due dates are predictions, not guarantees — the actual date of birth can vary by several weeks. Always consult your doctor or midwife for personalized care and dating.
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