Few dates in adult life carry as much weight as an estimated due date. It anchors prenatal schedules, maternity leave plans, baby showers and a long list of countdown traditions. The Pregnancy Calculator turns the first day of your last menstrual period into the answers everyone wants quickly: how many weeks along you are, how many days remain until the due date, which trimester you are in, and what share of the pregnancy has already been completed.
Counting From the Last Menstrual Period
Pregnancy is traditionally dated from the first day of the last menstrual period, or LMP. This may seem strange, since conception happens about two weeks later, but the LMP is the one date almost every woman can pin down reliably, and it works as a common reference point for all pregnancies. Under this convention, a pregnancy officially starts at day zero of the LMP and is described as being in its second week at the moment of conception. Delivery is conventionally placed at the 280th day — forty weeks. Ultrasound dating, which measures the embryo itself, usually lands within a few days of LMP-based dating when cycles are regular, which is why the method remains standard in clinics worldwide.
How the Calculator Works
You enter two dates plus one cycle setting. The LMP date is the start point of the pregnancy. The reference date defaults to today, but it can be moved to any date between 2000 and 2026, which makes the tool useful both for now and for planning or retrospective checks. The cycle length setting, normally 28 days, is used to refine the due-date estimate for people whose cycles run longer or shorter than average.
The engine first counts the exact number of whole calendar days between the two dates. From that single count every output is derived: the day count itself, the completed weeks, the standard and cycle-adjusted due-date countdowns, the trimester and the percentage of the pregnancy elapsed. Nothing else is estimated — the arithmetic is deterministic and transparent, with every formula shown on the page.
Reading the Results
The headline figure is weeks pregnant, computed as the day count divided by seven and rounded down. If the calculator reports 23 weeks and the day figure stands at 167, that remainder of six days carries real meaning in prenatal care, which is why the exact day count is shown separately. The days-to-due-date figure counts down from the classic 280-day full term, while the adjusted figure adds the difference between your cycle length and 28 days — a useful refinement if your periods are consistently longer or shorter than the median.
Below those, the trimester readout places the pregnancy in its first, second or third phase, and the progress gauge maps the completed share of the pregnancy onto the same three phases. Moving the LMP sliders even slightly shows how sensitive each figure is: a five-day shift in the LMP moves the week count, the countdown and the gauge band together.
The Trimesters at a Glance
The calculator splits the forty weeks into three blocks of about thirteen and a half weeks. The first trimester runs through day 97 — roughly the end of week 13 — and covers the period of organ formation, when most early scans and blood tests take place. The second trimester spans days 98 to 195, roughly weeks 14 through 27, and brings the first felt movements and the anatomy scan. The third trimester begins around week 28 and runs to delivery, covering the final growth phase and the run-up of antenatal appointments. These boundaries are convenient averages; real pregnancies develop on their own timelines, and clinical staff date each stage individually.
Why the Cycle Length Refines the Due Date
The 280-day rule assumes a textbook 28-day cycle. If your typical cycle is 31 days, ovulation — and therefore conception — tends to happen three days later than the assumption, so the likely due date shifts three days later as well. The adjusted countdown output applies exactly that correction: it adds the difference between your cycle length and 28 days to the standard countdown. For a 25-day cycle the correction goes negative, pulling the estimate earlier. Keep both figures in mind: the adjusted date is often the more realistic one, while the standard 280-day date is the one shared at clinics.
Understanding the Progress Percentage
The progress figure divides the days already completed by 280 and caps the result at one hundred. At day 140, exactly twenty weeks, the progress reads 50 percent. Around day 97 the marker passes the first-trimester boundary, and near day 196 it crosses into the third. The gauge bands follow the same cutoffs, so a single glance at the dial tells you both how far along you are and which phase of care you are in. During the final weeks the percentage approaches the cap while the days-to-due countdown shrinks toward zero — the two readouts form a natural pair for the last stretch.
A Worked Example
Take an LMP of January 15, 2026 and a reference date of August 8, 2026. The day count between the two dates is 205, so the calculator reports 205 days pregnant and 29 completed weeks. The standard countdown is 280 minus 205, or 75 days. With a 28-day cycle the adjusted figure matches the standard one; with a 31-day cycle it becomes 78 days. At 205 days the pregnancy sits past the 195-day boundary, so the trimester reads 3, and the progress gauge stands at about 73 percent — squarely inside the third-trimester band. Every one of those figures falls out of the single day count, which is the elegant core of the design.
Using the Calculator for Planning
The reference-date fields turn the tool into a planner as well as a tracker. To plan maternity leave around a future milestone, move the reference date forward and watch the week count rise and the countdown shrink. To reconstruct where a pregnancy stood at an earlier ultrasound, move the date backward. Because all results clamp at zero when the reference date precedes the LMP, experimenting with the sliders never produces negative values for the week count, countdown and progress — the tool simply waits until the pregnancy date has passed.
Reading the Edges of the Range
Some inputs place the calculator at the extremes of its range, and knowing what those look like is useful. With an LMP only a few days in the past, the week count reads 1 and the progress gauge sits near its bottom edge — perfectly normal for the earliest stage of a pregnancy. Around day 97 the trimester readout flips to 2, and the day-195 boundary usually coincides with the start of the final stretch of appointments. When the countdown reaches zero, the due-date figures stay at zero rather than counting up, so an overdue pregnancy is never miscounted, while the days-pregnant and week figures keep climbing as they should for as long as the reference date advances.
What This Calculator Does Not Do
The estimates here follow the standard rules, but real pregnancies differ. Only about four percent of babies arrive on their exact due date, with full term officially spanning weeks 37 through 42. Ovulation and conception timing vary even at the same cycle length, and an ultrasound in the first trimester is generally more precise than any LMP-based arithmetic. The calculator is also not a medical tool: it cannot flag risks, interpret symptoms or replace a midwife or obstetrician. Use it for orientation, planning and curiosity — and confirm anything clinically meaningful with your care team.
Common Questions About the Dates
Why is my week count different on other websites?
Different tools round differently. This calculator reports completed weeks — 205 days is 29 full weeks, not 30 — and always pairs the weeks figure with the exact day count so you can reconcile any other source.
What if my LMP date is only approximate?
The figures still orient you. Every output moves in lockstep, so a few uncertain days matter less for the trimester and the progress gauge than they do for the precise countdown. Let your care team cross-check the estimate at your first appointment.
Does the adjusted due date replace the standard one?
It refines it. For cycles near 28 days the two agree; the adjusted figure mostly matters for cycles at the extremes of the 21-to-35-day range, where the correction can reach a week. Compare both readouts before planning leave or a due-date celebration.
What if my LMP is in the future?
Set the reference date before the LMP and the results clamp to zero. Once the LMP date passes, the readings start counting up normally.
Key Assumptions
- Gestational age is measured from the first day of the last menstrual period, the standard obstetric convention; conception is treated as occurring about two weeks later.
- A full-term pregnancy is assumed to last exactly 280 days; real births between weeks 37 and 42 are considered normal.
- The adjusted due date shifts the standard estimate by (cycle length − 28) days.
- Results clamp to zero when the reference date precedes the LMP, and all dates are whole civil calendar days.
Every pregnancy has its own pace — due dates are averages, not appointments. Count the weeks, watch the gauge, and let your care team interpret what it means for you.