Pregnancy Calculator

What a pregnancy calculator does with the date you give it

A pregnancy calculator takes one date you already know and turns it into two numbers: an estimated due date and how far along you are today. The convention behind almost every calculator in the United States is a 280 day count from the first day of your last period. ACOG states that the estimated due date is 280 days after that first day, and that is the number this page starts from.

The count begins about two weeks before you conceived, which surprises people. Ovulation normally happens around day 14 of a 28 day cycle, so at fertilization the pregnancy is already logged as 2 weeks along. Those two weeks are bookkeeping, not biology. They exist because the first day of a period is a date most people can name, while the exact hour of fertilization is usually unknown.

Because the input is a date rather than a measurement, the output is a target and not an appointment. Only about 1 in 20 babies arrives on the exact date, and most arrive within the two weeks on either side of it.

What makes this page different from a plain due date tool is the input menu. You can start from a last period, a known conception or ovulation date, an IVF transfer with the embryo age, or a first trimester ultrasound. All four are reduced to the same equivalent last period, so the due date, your current weeks and the dated screening windows below stay consistent whichever route you pick.

Four ways to date a pregnancy, and which one wins

The routes are not equally reliable, and they are not meant to be averaged. Each one is a different way of estimating the same unknown, which is the day of fertilization. ACOG’s due date committee opinion is to set the date from the best available method, record it once, and then stop moving it.

ACOG is direct about the ranking. For a pregnancy that came from assisted reproduction, the date derived from the embryo and the transfer should be used. For everyone else, a first trimester ultrasound up to 13 weeks 6 days is the most accurate way to set or confirm gestational age, with crown to rump length accurate to about 5 to 7 days. A last period date is the starting point only when nothing better is available, because it assumes a 28 day cycle with ovulation on day 14.

In practice your provider compares the last period date with the earliest scan and keeps one of them. The scan does not automatically win; it wins when the gap is big enough to matter, and the size of that gap depends on when the scan was done.

The last period route: 280 days plus your cycle length

Start with the first day of bleeding, not the last. For a period that began on February 10, 2026 with a 28 day cycle, the count runs 280 days and lands on November 17, 2026. That is a Tuesday, and it is 40 weeks 0 days.

Cycle length matters because a longer cycle usually means later ovulation. The adjustment is simple: add one day for every day your cycle runs past 28. The same February 10 period on a 35 day cycle gives November 24, 2026, exactly 7 days later, because 280 + 7 = 287 days from the first day. A 24 day cycle pulls the date 4 days earlier. The calculator clamps the input between 20 and 45 days, since cycles outside that range make the arithmetic unreliable and a scan should set the date instead.

You may also see the calendar trick: subtract three months from the first day and add seven days. For February 10 that means November 10 plus 7, which is November 17, so the two methods agree here. They drift apart in other months. A March 14, 2026 period lands on December 21 by the calendar trick and on December 19 by a 280 day count, because the months in between are not all 30 or 31 days. Counting days is the sturdier method, and it is what this page does.

Irregular cycles are the weak spot. If your cycles swing by a week or more, the last period date can be off by more than a week, which is where a dating scan earns its place.

IVF transfer dating: 261, 263 or 260 days

An IVF pregnancy is the one case where the start is known exactly. The embryo has already been growing in the lab, so the transfer date carries a head start that has to be subtracted. ACOG gives the example directly: for a day 5 embryo the due date is 261 days from the transfer, and for a day 3 embryo it is 263 days.

The reason is the 266 day gap between fertilization and the due date. A day 5 transfer means fertilization happened 5 days earlier, so 266 - 5 = 261 days remain. On the day of a day 5 transfer you are already counted as 2 weeks 5 days pregnant, which is why the first positive test after a transfer can read as 4 weeks when the embryo is only days old.

Frozen embryo transfers use the same arithmetic: what matters is the age of the embryo, not whether the cycle was fresh or frozen, and not how many embryos were put back.

Dating from an early ultrasound

An ultrasound does not produce a due date directly. It measures the embryo, converts that measurement into a gestational age, and then the due date is the scan date plus whatever is left of the 280 days. A scan on April 6, 2026 that measures 7 weeks 2 days, which is 51 days, leaves 229 days: 280 - 51 = 229, so the due date is November 21, 2026.

Crown to rump length in the first trimester is the most accurate single measurement available, good to about 5 to 7 days. Accuracy falls as pregnancy advances, which is why a scan at 30 weeks is a poor tool for setting a date and a scan at 8 weeks is a good one.

When the scan and the last period disagree, ACOG uses a sliding threshold. A small gap is treated as noise and the last period date stands. A gap over the threshold means the scan takes over.

The thresholds are why a 4 day difference at 7 weeks usually changes nothing, while the same 4 day difference at 20 weeks is ignored by design. They also explain a common frustration: once a due date is set from the earliest good scan, later scans are not supposed to move it, even if the baby measures ahead or behind. Growth varies, dating does not.

The dated schedule that follows your due date

Once the due date is fixed, every scheduled test and shot becomes a date range rather than a vague month. Using the February 10, 2026 example with a November 17 due date, the calendar reads like this.

Two of these windows are worth memorizing. The Tdap shot is recommended in every pregnancy between 27 and 36 weeks, preferably early in that stretch, and CDC data put the drop in whooping cough risk for babies under 2 months at 78 percent. The group B strep swab is done at 36 or 37 weeks, and it is still done when a cesarean is planned, because labor can start before the scheduled date.

The anatomy scan sits at 18 to 22 weeks because before 18 weeks some structures are too small to image reliably, and after 22 weeks a finding leaves less time to plan. The glucose test at 24 to 28 weeks catches gestational diabetes early enough to act on it.

Reading the answer in weeks and days

Obstetric dates are written as weeks and days, so 36w5d means 36 weeks and 5 days, or 257 days of pregnancy. The shorthand matters because two appointments a week apart can look identical if you round to months. A pregnancy at 36w5d has 23 days left to a 40 week due date, which is a different situation from 32w0d with 56 days left.

Trimesters are fixed by ACOG at 0w0d to 13w6d for the first, 14w0d to 27w6d for the second, and 28w0d onward for the third. The boundaries fall on completed weeks, which is why the second trimester starts at 14w0d and not at the start of week 13.

Term is a separate set of labels and no longer covers everything from 37 to 42 weeks.

The old habit of calling anything from 37 to 42 weeks term is now discouraged, because babies born at 37 weeks have more feeding and breathing trouble than babies born at 39. If someone asks whether 37 weeks is full term, the accurate answer, per March of Dimes, is early term.

When two methods disagree, and when the date stops moving

Two dates that disagree by a few days are normal and do not need to be reconciled by hand. Your provider picks the better method, records one due date, and works from that. What you can do is arrive at the first visit with the dates you know: the first day of your last period, your typical cycle length, any ovulation tracking, and the transfer date and embryo age if this was IVF. Those four facts let the visit run on evidence instead of memory.

After a due date is established from a first trimester scan or from IVF, later scans should not reset it. ACOG describes a pregnancy with no confirming scan before 22 weeks as suboptimally dated, which is the clinical way of saying the date is a guess. If you are past 22 weeks and still unsure, that is a conversation for your provider rather than something a calculator can settle.

Twins follow the same dating math, since the due date describes the pregnancy and not the number of babies. What changes is the delivery plan, which is usually earlier and scheduled.

A final caution: a due date built from a last period assumes ovulation on day 14, and plenty of people ovulate on day 10 or day 20 without knowing. If your cycle is irregular, treat the last period route as a rough first pass and let an early scan set the date.