The formula
How it works
Track a pregnancy from the first day of the last period. The calculator estimates the due date, how many weeks along you are now, the current trimester and how many days remain.
FAQ
Why is the due date measured from the last period?
Because the exact date of conception is rarely known, doctors date a pregnancy from the first day of the last menstrual period, which is easier to pin down. Full term is counted as 40 weeks — 280 days — from that date, even though conception happens about two weeks later.
How accurate is the estimate?
It assumes a regular 28-day cycle with ovulation on day 14, so it is a solid estimate rather than a certainty. Only about 4% of babies arrive exactly on the due date; most come within a week or two either side. An ultrasound scan gives a more precise date.
What defines each trimester?
This calculator splits the pregnancy roughly at week 13 and week 27 — the first trimester runs to week 13, the second to week 27, and the third to the due date. These boundaries are a common convention rather than a fixed medical rule, and different sources round them slightly differently.
What is the difference between gestational age and fetal age?
Gestational age is counted from the last menstrual period, which is what this calculator uses, while fetal (or conceptional) age is counted from the estimated conception, typically about two weeks later. So a baby described as “10 weeks” gestational age is usually closer to 8 weeks post-conception.
Does an irregular cycle change the result?
Yes — the 280-day estimate assumes ovulation around day 14 of a 28-day cycle, so longer or irregular cycles shift the actual conception date and make the due date less reliable. If your cycles vary a lot, an early ultrasound is generally used to refine the estimate.
How is the progress percentage calculated?
It is simply the number of days since the last menstrual period divided by the full 280-day term, shown as a percentage. It is a rough visual guide to how far along you are, not a separate clinical measurement.
Why might my due date change after an ultrasound?
Early ultrasound measurements of the fetus can be more accurate than the last-period method, especially for irregular cycles, so care providers sometimes adjust the due date based on the scan. If that happens, the scan-based date generally takes precedence over the one calculated here.
About the pregnancy calculator
This calculator tracks where a pregnancy stands based on the first day of the last menstrual period. From that single date it estimates the due date, the current gestational age in weeks and days, which trimester you are in, and how long is left. It is the standard way pregnancies are dated, giving an at-a-glance picture of progress without needing a scan.
How to use it
Enter the first day of your last menstrual period. The calculator shows your estimated due date — 40 weeks later — along with how many weeks and days pregnant you are today, your trimester, and the days remaining. As time passes, the same date keeps the progress figures up to date. For irregular cycles, treat the results as an approximation.
The formula
The due date is , where LMP is the first day of the last period and 280 days is 40 weeks. Current gestational age is simply today’s date minus the LMP, expressed in weeks and days. The three trimesters split roughly at weeks 13 and 27, and the days remaining count down from today to the due date.
Where it is used
Expectant parents use it to follow the pregnancy week by week, plan appointments and prepare for the birth, while it also underpins the timing of scans and screening tests. Because everything keys off one easily remembered date, it is the quickest way to answer “how far along am I?” between medical visits. It pairs naturally with ovulation and due date calculators.