Where the due date and week count come from

What a pregnancy week and due date are

MedlinePlus (U.S. National Library of Medicine) says a normal pregnancy lasts around 10 months and has 3 trimesters. This app's name, Yeoldal, is the Korean phrase for those ten months.

A pregnancy week isn't a calendar month split into four; it's the number of completed weeks and days since the first day of your last period. If today is day 143 since that date, you're 20 weeks and 3 days along.

The due date is a statistical reference point, not a guaranteed day. Real delivery dates vary from person to person, and this calculator's number gives way to an ultrasound diagnosis whenever you have one.

The standard calculation

The U.S. Department of Health and Human Services' Office on Women's Health says pregnancy lasts about 40 weeks, counting from the first day of your last normal period. So the due date is the first day of your last period plus 280 days. Ovulation and conception actually happen about two weeks after that date, so those 280 days include two weeks before conception. That's the standard convention.

The trimester ranges come from the same source: first trimester week 1 through week 12, second trimester week 13 through week 28, third trimester week 29 through week 40. Because this app counts completed weeks and days, it treats week 13, day 0 as the start of the second trimester and week 29, day 0 as the start of the third.

The same source also gives full-term: "At 39 weeks, your baby is considered full-term." A birth before that point is what people call preterm.

What this calculator does

Here is what the code does, in order. There are three ways to enter a starting point: the first day of your last period directly, the weeks your doctor gave you plus the date of that appointment, or the due date your doctor gave you. All three collapse to one reference date internally. Entering the period date makes that the reference date; entering weeks subtracts that many days from the appointment date; entering a due date subtracts 280 days from it.

Once there's a reference date, the calculator counts the days from it to today and splits that into weeks and days. Past 13 weeks (91 days) it's the second trimester, past 29 weeks (203 days) it's the third. The due date is the reference date plus 280 days, and the days left is the due date minus today (negative once it has passed).

The weekly milestone text only uses the weeks the source itself named: 4, 8, 12, 16, 20, 24, 32, and 37. Weeks in between show the most recent milestone that has passed, and before week 4 there is none shown. The app does not invent development details for weeks the source didn't cover.

If the reference date works out to be after today (a future period date or appointment), the calculator refuses and says why. If it's more than 45 weeks in the past, it also asks you to check the date. That's well outside how long a human pregnancy can run.

Copying the result link just puts the numbers you entered into the address as parameters. Nothing is stored on a server, and whoever opens the link gets the same calculation redone on their own screen.

Filling it in

The first day of your last period is when it started, not when it ended. If you're not sure of the exact date, a rough one still gives a calculation, though the further your real ovulation date is from average, the bigger the error.

If your cycle is irregular or you don't know the date, the weeks your doctor gave you from an ultrasound are better grounded. Enter those weeks with the date they were measured, and this calculator works back to the same reference date.

If you already have a due date from your doctor, enter that directly. Whichever mode you use, the formula is the same, and only the direction of working out the reference date changes.

When it differs from your doctor

MedlinePlus says you should have an ultrasound done in the first trimester to get an idea of your due date. An ultrasound measurement of the baby is often closer to the real conception date, so the value this calculator gives from your last period alone can differ from what your doctor tells you.

When that happens, follow the doctor's number. Since this calculator accepts all three kinds of input, entering the weeks or due date your doctor gave you will recompute everything else on this screen (today's week count, trimester, and milestone) from that same figure.

If your cycle runs much shorter or longer than 28 days, a due date calculated from the last period alone carries a bigger error, because this calculator doesn't take cycle length as a separate input. In that case a doctor's estimate is better grounded.

Where people get stuck

The week and day count can be off by a day or two from what your doctor said. That's ordinary, since an ultrasound measurement and a last-period calculation use different methods. A day or two isn't something to worry about; if the gap is much bigger, ask your doctor.

If the due date has passed and you haven't given birth, this calculator only states that fact and doesn't judge it. Going past the due date isn't rare, and what happens next is for your clinic to decide.

For multiple pregnancies, or an IVF pregnancy where the conception date is known precisely, this calculator's standard formula may not fit as well. In that case, use the due-date mode with the figure your clinic gave you, just to check today's week count.

The milestone text describes a commonly published sequence, not a diagnosis of your pregnancy. If something feels concerning, that's a question for your doctor, not this screen.