Find your due date and see exactly how many weeks pregnant you are — from your last period, a due date you already have, an IVF transfer, an ultrasound scan or the date you conceived.
5 Ways to Calculate
Week-by-Week Timeline
Instant & Private
What Do You Know?
The first day you bled, not the day it ended.
28 days
Leave this at 28 if you are not sure. Most calculators assume it.
Works backwards to tell you how many weeks along you are today.
A day-5 blastocyst is the most common transfer. Your clinic will have told you which.
Use this only if you know your ovulation date. Otherwise the last-period method is more reliable.
weeks
days
Copy both numbers straight off the scan report. This is the most accurate method there is.
Estimated due date
—
How far along
—
Trimester
—
Months
—
Countdown
—
Progress0%
Twins: the due date above is still calculated the standard way, but
twin pregnancies commonly deliver earlier — often around
—. Your clinic will monitor this closely.
Key Dates
Pregnancy counted from—
Estimated conception—
Second trimester starts—
Third trimester starts—
Full term begins (37 weeks)—
Week-by-Week Timeline
your dates, not generic weeks
This is an estimate, not a diagnosis. Only about 4% of babies arrive on their
exact due date, and most arrive somewhere in the weeks either side of it. A dating scan in the
first trimester is more accurate than any calculation from a date, so if your midwife or doctor
has given you a different due date, use theirs. This tool is for information only and does not
replace medical advice.
No sign-up, no email — the dates you enter are never saved or sent anywhere.
This pregnancy calculator answers the two questions people actually arrive
with: when is the baby due, and how far along am I right now. Pick whichever date you already
have — you do not need all five.
Last period. The standard method. Enter the first day of bleeding and the tool applies Naegele's rule, adding 280 days. This is how to calculate pregnancy week by LMP, and it is what a clinic uses before your first scan.
Due date. If you already have a due date, this works backwards. Use it to calculate pregnancy by due date and see how many weeks along you are today.
IVF transfer. Enter the transfer date and pick day 3, 5 or 6. A pregnancy calculator due date IVF result is exact at the front end, because the date of fertilisation is known rather than estimated.
Conception. If you tracked ovulation, enter that date to calculate pregnancy from conception.
Ultrasound. Enter the scan date and the weeks and days it reported. This is how to calculate pregnancy by ultrasound, and for a natural conception it is the method clinics trust most.
Whichever you choose, the result updates as you type. Nothing is submitted and there is no
sign-up — it is a pregnancy calculator online that runs entirely in your browser.
How many weeks pregnant am I?
This is where most calculators quietly confuse people, so it is worth being clear.
Pregnancy is counted from the first day of your last period, not from conception.
Conception happens around two weeks later, which means on the day you conceive you are already
described as two weeks pregnant.
It sounds wrong the first time you hear it. The reason is practical: almost nobody knows the exact
day they conceived, but most people know when their last period started. That one decision is
where every other number in pregnancy comes from — the weeks, the months, the trimester lines —
and it is worth understanding properly:
Pregnancy Week
by Week — Why You Are Two Weeks Pregnant Before You Conceive. So the whole system is
anchored to a date that can actually be remembered. When you ask a
pregnancy calculator how many weeks am I, the answer you get — and the one your
midwife uses — is measured that way.
The tool shows your pregnancy in months and weeks together for the same reason.
Weeks are what clinics use; months are what everyone else asks about. Both are on screen so you
do not have to convert in your head.
How the due date is worked out
Every method here resolves to a single due date first, and the weeks, trimester and timeline are
all derived from it. That is deliberate — it guarantees the tool can never tell you that you are
40 weeks along on a day that is not your due date.
If you know
The calculation
Last period (LMP)
LMP + 280 days, adjusted if your cycle is not 28 days
Conception date
Conception + 266 days
IVF, day-5 transfer
Transfer + 261 days
IVF, day-3 transfer
Transfer + 263 days
An ultrasound scan
Scan date minus the weeks and days it reported, then + 280 days
A due date already
Counted backwards 280 days to find the start
Cycle length, and why it moves your date
Naegele's rule assumes a 28-day cycle with ovulation on day 14. Plenty of people do not have
one. If your cycle runs 35 days, ovulation is roughly a week later, so conception is a week
later and your due date moves back by about a week.
The slider handles this. Leave it at 28 if you are unsure — that is what the NHS and most
hospitals assume, so your result will match the date they give you. Adjust it only if you know
your own cycle well.
Your pregnancy timeline
The pregnancy timeline below the results turns generic milestones into your
actual calendar dates. Instead of reading that the anomaly scan happens around week 20, you see
the date that falls on for you. Weeks already behind you are dimmed, and the week you are in now
is highlighted — a pregnancy calendar and a
week-by-week view in one place.
The trimesters break at fixed points: the first runs to the end of week 13, the second covers
weeks 14 to 27, and the third starts at week 28. Full term begins at 37 weeks. Those boundaries
are shown as dates in Key Dates, so you can see exactly when each one arrives.
How accurate is a due date?
The arithmetic is exact. The prediction is not, and no calculator can change that. Only about
4% of babies are born on their due date, and a normal term delivery lands
anywhere between 37 and 42 weeks. The due date is best read as the centre of a range rather than
an appointment.
An early dating scan beats any of these methods, because it measures the baby directly instead
of inferring from a remembered date. If your scan date and your LMP date disagree by more than
about a week, clinics normally go with the scan. If you are expecting twins, the calculation is
unchanged, but delivery commonly comes earlier — tick the twins box and the tool shows that
range alongside your date.
Frequently Asked Questions
It applies Naegele's rule: 280 days, or 40 weeks, from the first day of your last menstrual period. This calculator lets you start from five different dates — your last period, a due date you were already given, an IVF embryo transfer, an ultrasound scan, or a conception date — and each one resolves to the same due date figure, from which the weeks, trimester and timeline are then derived.
Enter any one of the five dates and the result appears immediately as weeks and days, for example 12 weeks and 3 days. Remember that pregnancy is counted from the first day of your last period rather than from conception, so you are already considered two weeks pregnant on the day you actually conceive. That is the convention clinics use, so the number here matches what your midwife will tell you.
Yes. Switch to the Due Date tab and enter the date you were given. The tool counts back 280 days to find the start of the pregnancy and then works out how many weeks and days along you are today, which trimester you are in, and how long is left. This is the quickest option if a scan has already given you a date.
From the transfer date plus 266 days, minus the age of the embryo when it was transferred. A day-5 blastocyst gives transfer plus 261 days, and a day-3 embryo gives transfer plus 263 days. IVF dating is exact at the front end, because the moment of fertilisation is known instead of being estimated from a period date.
Yes, and for a natural conception it is the most accurate option here. Choose the Ultrasound tab, enter the date of the scan and the weeks and days the report gave, and the tool winds that back to the start of the pregnancy before adding a full term. Because the measurement is taken from the baby rather than from a remembered date, clinics normally use the scan date whenever it disagrees with the last-period date by more than about a week.
Use the cycle length slider. A longer cycle means you ovulate later, so conception and the due date both shift later by roughly the same number of days. If you are not sure of your cycle length, leave it at 28, because that is the assumption most hospitals use and it will match the date they give you.
Only about 4% of babies are born on their exact due date, and a normal term birth happens any time between 37 and 42 weeks. Treat the date as the middle of a range rather than a fixed appointment. A first-trimester dating scan is more accurate than any date-based calculation, so if your scan gives a different date, use that one.
The calculator shows months alongside weeks and days. Months are less precise than weeks because the nine months of pregnancy do not divide evenly into 40 weeks, which is why clinics count in weeks instead. The mapping used here follows the standard one: month one covers weeks 0 to 4, month three covers weeks 9 to 13, and month nine covers weeks 36 to 40.
The due date is calculated the same way for twins as for a single baby, so the date shown is correct. Twin pregnancies do tend to deliver earlier though, commonly between 35 and 37 weeks. Tick the twins box and the tool displays that likely range next to your date, but your clinic will monitor a twin pregnancy far more closely than any calculator can account for.
No. Every calculation runs in your browser and nothing you type leaves your device. There is no account, no upload and no tracking of the dates you enter, which matters more here than on most tools because a pregnancy date is genuinely private information.