Pregnancy week calculator: from period, ovulation or transfer

Three ways to the same number

You can calculate your pregnancy week in three ways, depending on which starting date you know: the first day of your last period, the day of your ovulation, or, in the case of IVF, the day of the transfer. In the end, you always get the same kind of number, a week and a day, for example 12+3. But the three calculations start from different points, which is why two methods can differ by a few days without either one being wrong.

From your last period

The most common method is Naegele's rule. It counts from the first day of your last period, not from the day of conception. Week 0+0 is therefore the first day of bleeding, even though no baby exists yet at that point. Your due date falls 280 days, or 40 weeks, later.

This calculation assumes a 28-day cycle with ovulation around day 14. That doesn't hold true for every woman. If your cycle is longer, ovulation happens later, and the calculated week ends up ahead of where your baby actually is. If it's shorter, it's the other way around. Even so, this is the method entered as standard in the German maternity record (Mutterpass), because the first day of your last period is usually the only date everyone knows, even without a temperature chart or cycle app.

From ovulation

If you know your ovulation date, for example through temperature charting, ovulation tests, or an app that tracks your cycle over several months, you can calculate more precisely. From ovulation or conception to your due date is 266 days, or 38 weeks. This calculation sidesteps the problem of varying cycle lengths because it starts directly from the event that actually matters: conception.

The difference from the period-based calculation matters most with irregular cycles. With a 35-day cycle, ovulation might not happen until day 21, a week later than the standard assumption. The two calculations can then easily be a week apart without either being wrong. Both are estimates, just with different starting points.

From the transfer date

With IVF, the starting date is known as precisely as it ever gets: the day the eggs were retrieved essentially counts as the day of conception. At transfer, the embryo's age is known, usually three or five days. From a day-three transfer to your due date is 263 days; from a day-five transfer, the blastocyst stage, it's 261 days.

This method is considered the most accurate of all, because there's no ovulation to estimate. The fertility clinic usually enters the due date directly in your paperwork, which you then transfer into your maternity record (Mutterpass).

What 12+3 means

The notation with the plus sign often confuses people at first. Week 12+3 means 12 full weeks have passed, plus 3 more days. Together that's 87 days since the starting point of the calculation, not since conception. That means you're already in your 13th week of pregnancy, because weeks are only counted once they're complete, much like a birthday: on a child's first birthday, they turn one year old, rather than still being in their first year of life.

This way of counting is common in Germany, Austria and Switzerland. In English-language sources you'll often see just "week 13" without the days, which can lead to confusion when you're comparing figures. So if you compare a number from an app or a forum with your maternity record (Mutterpass), it's worth taking a second look at whether it means weeks alone, or weeks and days.

Why the ultrasound shows a different number

In the early weeks, your doctor measures the crown-rump length during the ultrasound, meaning the length of the embryo from head to bottom. At this early stage, embryos grow very uniformly, regardless of how tall the parents will later be. That's why this length allows the age to be read off fairly precisely between the eighth and twelfth week, often more accurately than from your last period, especially if your cycle was irregular or ovulation happened later than assumed.

If the measured size differs from the calculated week by more than about five to seven days, your doctor's office will adjust the due date to match the ultrasound measurement. This is then noted in your maternity record (Mutterpass), often with a note on which method was used to determine the date. From the middle of pregnancy onward, this correction is no longer made, because from that point on, babies grow at different rates depending on their individual disposition. A discrepancy at week 26, then, doesn't automatically mean the date is wrong; it's more a sign of how big your baby is compared to the average.

That's why the due date set by ultrasound in the first trimester usually stays in place until birth, even if later measurements suggest something different. Doctors only change the due date again in exceptional cases.

Why the exact week matters in practice

Getting the week right isn't just a formality. The first-trimester screening between weeks 11 and 14, the glucose tolerance test around weeks 24 to 28, and the question of when labor should be induced, all depend on as accurate a date as possible. The further off the calculated week is, the harder it becomes to time these checks correctly. That's why it's worth bringing all the dates you have to your first prenatal appointment: the first day of your last period, any temperature charts you've recorded, or the transfer report from your fertility clinic.

What the number ultimately means

Whichever method you use, the due date remains an estimate, not a fixed day. Only a small percentage of babies are actually born on that exact date. Still, knowing the week is helpful, because it shows which developmental milestones are typical for this stage, from your baby's size to the check-ups that are coming up.

Lunabia lets you calculate based on either your last period or the due date you got from your ultrasound, and shows you the week and day just as you'd find them in your maternity record (Mutterpass). Which method you enter is up to you, depending on which date you trust the most.