How many weeks pregnant am I?
You're counting backwards on your fingers and the number doesn't make sense. Everyone keeps saying a number that seems to include time before you were pregnant.
You're not the first person to ask.
Pregnancy is counted from the first day of your last period, not from the day you conceived. Which means you were counted as two weeks pregnant before you were pregnant at all. Count from that first day to today. That's how many weeks you are.
Why is pregnancy counted from my last period?
It doesn't make much sense in modern terms. We know you aren't pregnant when you're bleeding. Quite the opposite.
But think about how long people have been counting pregnancies. The first day of your last period was the last clear external sign there was to count from. For most of history, pregnancy wasn't really confirmed until a woman felt the baby move.
We're in an unusual moment historically. We can see inside, we can measure, we can date a pregnancy within a few days. The counting convention is older than any of that, and it stuck.
So yes, the first two weeks of your pregnancy are two weeks you weren't pregnant. Everyone knows. It's counted that way anyway, everywhere in the world.
If it's been four weeks since the first day of your last period, you're four weeks pregnant. You may have only just found out. Some early tests won't pick it up yet.
If you're eight weeks pregnant, your baby is about six weeks old.
How do I work out my due date?
If your cycle is regular and around 28 days, the calculation is simple. Take the first day of your last period, add nine months, then add seven days.
So if your last period started on 1 January 2026, nine months takes you to 1 October. Add seven days. Your estimated due date is 8 October 2026.
A regular cycle doesn't mean how many days you bleed. It means the number of days from the first day of one period to the first day of the next, and whether that number is much the same each month.
If your cycle is regular but longer or shorter than 28 days, adjust by the difference.
30-day cycle: add two days. In the example above, that moves the due date to 10 October.
27-day cycle: subtract one day. That moves it to 7 October.
What if my cycle is irregular?
This is where it gets less tidy, and the honest answer is that it depends how irregular.
If your cycle moves around a bit but sits in a narrow range, say 27 to 29 days, you might decide that's close enough. A day or two either way isn't going to change much about your care.
If it swings more than that, or you genuinely couldn't say what your usual cycle length is, the usual recommendation is a dating ultrasound. Ideally around eight weeks.
There's a circularity in that, of course. If you don't know how pregnant you are, it's hard to book a scan for the right week. What usually happens is you sit down with your midwife or GP at your first appointment, work out a best estimate together, and plan the scan from there.
You might also want to read: What tests will I be offered in early pregnancy?
What if I don't know when my last period was?
Same answer. A dating ultrasound is usually what's recommended.
Most women don't know the exact date but do have a sense of roughly when it was. Bring your best guess. Work it out with your midwife or GP. A rough estimate is enough to plan a scan for about the right week, and the scan does the rest.
How does dating work with IVF?
Differently, and more precisely.
With IVF we know the age of the embryo and the date it was transferred. That's better information than a period date or a measurement, so it's what we use, regardless of what any later scan says.
Usually the embryo is five days old at transfer, but your clinic will have the exact number. Most women who've been through IVF know their dates rather well by this point.
Why did my due date change after the scan?
Two reasons.
The first is that the scan was more accurate than what you had. If you weren't sure of your dates to begin with, the scan gives a better estimate and that becomes your due date. Generally the first ultrasound after about seven weeks is the one we settle on.
Later scans usually don't change it. As pregnancy goes on, genetics takes over, which is why some babies are born at two and a half kilos and others at four and a half. A big baby at 32 weeks isn't a differently dated baby. It's a big baby.
The second is a discrepancy between your dates and the measurements. This is the one worth understanding, because a difference on the screen doesn't automatically mean your due date changes.
There's an allowance built in. In early pregnancy, up to about nine weeks, a difference of five days or less between your dates and the scan is considered acceptable, and your original date usually stands. From nine weeks onwards the allowance widens to about a week. Only once the gap is bigger than that do we start talking about changing the date.
One thing worth knowing. If you have several scans before twelve weeks, which isn't unusual, you can arrive at your booking appointment with three different due dates and no idea which one is real. This is worth sorting out early. Sit down with your care provider, look at your particular circumstances, and settle on one date.
Generally we find the date early and it doesn't change for the rest of the pregnancy. That's harder if you've seen a lot of different people and nobody explained which date they were using, or why.
How accurate is a due date, really?
Two questions hiding inside one.
How accurately can we date a pregnancy? Quite accurately, in early pregnancy. Within a few days. That accuracy drops the further along you are, for the same reason as above: babies grow at different rates once genetics gets involved.
How often do babies arrive on their due date? Rarely.
A due date is the middle of a range, not an appointment. Term is considered anywhere from 37 to 42 weeks, and most babies come somewhere in that stretch. Research following women with well-dated pregnancies has found the median birth falls a few days after forty weeks, not on it.
Which is worth knowing in advance, before you're 40 weeks and 3 days and everyone you know is texting.
What does "six weeks and four days" mean?
It means that counting from the first day of your last period, or from what the scan measured, you are six weeks and four days into your pregnancy.
Your baby is about four weeks and four days from conception.
Both numbers are correct. They're just counting from different places.
One thing worth doing
Write down the first day of your last period, and whether your cycle is usually regular.
It's the first thing anyone will ask you, and by the time you're asked, most women can't remember.
This is general information about pregnancy, not advice about you. It doesn't replace the care of your own midwife, GP or maternity service, and it can't account for what's particular about your pregnancy.
If you're worried about yourself or your baby, contact your maternity service or go to your nearest emergency department. Don't wait to be sure.
