How Many Weeks Pregnant Am I? A Simple Guide to Dating Your Pregnancy
You count pregnancy weeks from the first day of your last menstrual period, not from the day you conceived. So if your last period started six weeks ago, you’re six weeks pregnant — even though conception happened around two weeks after that date. A full pregnancy runs 40 weeks, or 280 days, from that first day.
That single quirk confuses almost everyone. It’s the reason a woman who took a positive test yesterday can walk out of a clinic being told she’s already five weeks along.
If you’d rather not do the arithmetic yourself, our Pregnancy Week Calculator works it out in a few seconds. Enter one date and it tells you how many weeks and days pregnant you are, your estimated due date, which trimester you’re in, and how big your baby is this week.
Why pregnancy is counted from your last period
It seems illogical at first. You weren’t pregnant during your period, so why start counting there?
The honest answer is that it’s the only date most women can pin down reliably. Ovulation is invisible without tracking. Conception can happen up to five days after intercourse, because sperm survive that long. But the day your last period started is a date you can usually name.
So doctors worldwide agreed on it as the starting line. It’s called your LMP — last menstrual period — and every scan, every growth chart and every due date is measured from it.
The practical effect is that the first two weeks of your “pregnancy” happened before you were pregnant at all. Week 1 is your period. Week 2 is the run-up to ovulation. Conception usually lands somewhere in week 3.
This is also why a home test can only turn positive around week 4 or 5, when there’s enough hCG in your system to detect.
Four ways to work out how far along you are
Not everyone has a tidy 28-day cycle or a remembered date. Our calculator handles four different starting points.
1. From your last menstrual period
The standard method, and the one your obstetrician will use. Enter the first day of your last period — the first day of proper bleeding, not spotting.
If your cycles run longer or shorter than 28 days, enter your average cycle length too. A woman with a 35-day cycle ovulates around day 21 rather than day 14, which shifts her due date about a week later. Skipping this step is one of the most common reasons a home-calculated due date comes out wrong.
2. From your conception or ovulation date
Useful if you’ve been tracking. If you’ve used ovulation strips, monitored your basal temperature, or used our Bloom fertile window tracker, you may know almost exactly when you conceived.
The calculator works backwards, treating conception as day 14 of your cycle, then dates the pregnancy from there.
3. From a due date you already know
If a scan has already given you a due date and you simply want to know what week you’re in today, this is the quickest route. Enter the due date and it reverses the calculation.
4. From your IVF embryo transfer
This one matters a great deal to our patients, and most general calculators get it wrong or don’t offer it at all.
IVF dating is actually more precise than natural conception dating, because the exact age of the embryo is known to the day. There’s no guesswork about when ovulation happened.
- A Day 5 transfer (blastocyst): due date is the transfer date plus 261 days
- A Day 3 transfer (cleavage stage): due date is the transfer date plus 263 days
Select the IVF option, enter your transfer date, choose Day 3 or Day 5, and the calculator applies the correct figure. If you’ve been through treatment at DFMC or elsewhere, this will give you a far more accurate date than an LMP-based estimate.
How your due date is actually calculated
The formula is called Naegele’s rule, and it’s been in use since the early 1800s. Take the first day of your last period, add one year, subtract three months, then add seven days. The result is 280 days — 40 weeks — from where you started.
An example. If your last period began on 10 March 2026, your estimated due date is 15 December 2026.
Now the part nobody tells you clearly enough: only about 1 in 20 babies arrive on their exact due date.
Most babies are born somewhere between 37 and 42 weeks. Your due date is a reference point for scheduling scans and tracking growth. It is not an appointment, and going past it is normal, particularly with a first baby.
It helps to think of it as a due month rather than a due date. Most of the anxiety we see in the final weeks comes from treating that one day as a deadline.
Which trimester am I in?
| Trimester | Weeks | What’s happening |
|---|---|---|
| First | 1 – 13 | Organs, brain and spine form. Nausea and fatigue are common. Dating scan usually done here. |
| Second | 14 – 27 | Energy often returns. First movements felt. Anomaly scan around week 20. |
| Third | 28 – 40 | Rapid weight gain. Baby settles head-down. Regular growth checks. |
Terminology you’ll hear near the end:
- Early term: 37 weeks to 38 weeks 6 days
- Full term: 39 weeks to 40 weeks 6 days
- Late term: 41 weeks
- Post-term: 42 weeks and beyond
Anything from 37 weeks onward is considered safe for delivery in a healthy pregnancy.
What your baby is doing right now
Every week brings something new, and it’s genuinely lovely to follow along.
At week 8, fingers and toes are forming and the heart is beating steadily. By week 12, your baby has fingerprints. At week 16, you may feel the first flutters. Week 20 is the halfway mark and usually the detailed anomaly scan. By week 24, the lungs are building the tiny air sacs they’ll one day breathe with. At week 28, your baby starts having dream sleep. By week 34, they’re likely head-down and nearly ready.
Our calculator includes a week-by-week view for every week from 4 to 40, with a size comparison, what’s developing, what to eat, safe movement for that stage, and a note for you.
When a scan gives a more accurate date
A calculator is an excellent starting point. A dating scan is the gold standard.
Between roughly 8 and 13 weeks, a sonographer measures your baby from crown to rump. At this stage all babies grow at a very similar rate, so that measurement dates the pregnancy to within a few days. Later scans are less reliable for dating, because babies begin growing at their own individual pace.
You should book a scan and see a doctor if:
- Your cycles are irregular and you can’t be confident of your LMP
- You conceived while on contraception or soon after stopping it
- You conceived through IVF, IUI or any fertility treatment
- You have bleeding, cramping or severe pain
- You have a medical condition such as PCOS, diabetes, thyroid disease or high blood pressure
- You’ve had a previous miscarriage or a high-risk pregnancy
At Dhiya Fertility & Maternity Center in Chennai, our early pregnancy scans confirm the location and viability of the pregnancy, check the heartbeat, and give you a due date you can rely on for the rest of your antenatal care.
Common questions
Can I be pregnant and have a normal-looking period? Some women have light spotting in early pregnancy, sometimes around the time a period was due, which is often implantation bleeding. It’s usually lighter and shorter than a real period. A proper full flow is not typical, so if you’re unsure, take a test and speak to your doctor.
My cycles are irregular. Will the calculator still work? It will give you an estimate, but the margin of error is wider. Enter your average cycle length for a better result, then confirm with a dating scan. Irregular cycles are common with PCOS and are one of the clearest reasons to get an early scan.
Does the calculator work after IVF? Yes, and it’s more accurate for IVF than for natural conception. Choose the IVF option, enter your embryo transfer date, and select Day 3 or Day 5.
How many weeks pregnant am I if I conceived last month? Add roughly two weeks to the time since conception. If you conceived four weeks ago, you’re around six weeks pregnant.
Is 40 weeks nine months? Not quite. Forty weeks works out to about nine months and one week, because calendar months are longer than four weeks. This is why the month count and the week count never quite line up.
When should I book my first antenatal appointment? As soon as you have a positive test, ideally before week 8. Early care matters most in the first trimester, when your baby’s major organs are forming.
Try the calculator
Enter one date and get your week, your due date, your trimester and a week-by-week look at your baby’s growth — free, and no sign-up.
Open the Pregnancy Week Calculator →
And when you’re ready for care, our maternity and fertility team in Chennai is here for the whole journey, from your first scan to the day you hold your baby.
