Due date calculator and week-by-week tracker

How to use the tracker

Pick the date you know. Most couples have a due date from her provider by the first or second appointment; use that if you have it, because it is usually based on an ultrasound. Otherwise use the first day of her last period, a known conception date, or an IVF transfer date.

You get three things:

1. Where you are: weeks and days, the trimester, and a countdown.
2. What the baby is up to: a rough size and one headline development for the week.
3. What to do: three to five specific jobs for you this week, plus a link to the full week guide with the reasoning and sources.

The tracker remembers your date on this device, so next week you can just open the page. The link in your address bar also updates. Send it to your partner and it opens on the same week.

The math, briefly

Obstetricians date pregnancy from the first day of the last menstrual period (LMP), not from conception. The classic formula, Naegele’s rule, adds 280 days to the LMP. That assumes ovulation on day 14 of a 28-day cycle, so the calculator nudges the date later for longer cycles and earlier for shorter ones.

That LMP date gets checked against an early ultrasound. According to ACOG, a first-trimester measurement of the embryo (crown-rump length) is the most accurate method, and if it disagrees with the period date by more than a set margin, the ultrasound date wins. Once the due date is set, it generally should not change based on later scans.

For IVF pregnancies, the date is exact because the embryo’s age is known: a day-5 embryo transfer date plus 261 days, or a day-3 transfer plus 263 days.

What “term” means

Babies do not read calendars. ACOG defines the ranges like this:

Term	Gestational age
Early term	37 weeks 0 days to 38 weeks 6 days
Full term	39 weeks 0 days to 40 weeks 6 days
Late term	41 weeks 0 days to 41 weeks 6 days
Post-term	42 weeks 0 days and beyond

Practically, that means your bags should be packed and the car seat installed by 36 weeks, and your work should know that “the due date” means “any day in a five-week window.”

Using the weekly to-do list

Each week’s list was written for that week. Early on it is about the news, the first appointment, and how to be useful when she feels awful. In the middle months it is money, leave, and gear. Late in the third trimester it is the hospital bag, the route to the hospital, and getting your work handover done.

None of it is mandatory. If you only do one thing a week, you will still arrive at delivery day better prepared than most. If you want the plan in one printable place, that is what the Playbook is for.

When the dates are off

If the tracker says something that does not match what her provider told you, believe the provider. Common reasons for a mismatch: the due date was moved after an ultrasound, her cycles are irregular, or you entered the period date when you meant the due date. Change the method in the drop-down and try again.