How Many Weeks Pregnant?
Find out exactly how far along you are — in weeks, days and trimester. See what's happening with your baby this week and which scans are due next.
Work out how many weeks pregnant you are
Pick the date you know. The calculator does the rest.
Cycle length adjustment (for irregular or long/short cycles)
Your baby this week
Enter your dates above to see what's developing this week.
The next few weeks
How gestational age is calculated
Gestational age is the number of weeks and days since the first day of your last menstrual period (LMP). This is the clinical convention used worldwide — your midwife, sonographer and obstetrician all measure pregnancy from LMP, not from conception. A typical pregnancy lasts 40 weeks (280 days) measured this way.
Because ovulation and conception happen roughly two weeks after LMP, gestational age is always about two weeks longer than the actual age of the embryo. A "six-week" pregnancy contains a four-week-old embryo. This trips up almost everyone at first — but it keeps pregnancy dating consistent between women who do and don't know their exact conception date.
If you had IVF, the calculator uses your transfer date and embryo age (day-3 or day-5) to work backward to an equivalent LMP. IVF gives the most accurate gestational age of any method because fertilisation happened in the laboratory on a known day. For irregular cycles, the cycle-length adjustment shifts your notional LMP to reflect later or earlier ovulation.
Why weeks and days?
Pregnancy is measured in weeks + days (e.g. 14w 3d) rather than months because every major clinical milestone is week-specific. The anomaly scan window, for example, opens at 18w 0d and closes at 22w 6d.
Three months doesn't mean the same thing as 12 weeks — months have 28–31 days and pregnancy decisions need the precision that days-based gestational age provides.
When your gestational age matters most
Gestational age isn't just a number — it's the trigger for every clinical decision during pregnancy.
Frequently asked questions
What does gestational age actually mean?
Gestational age is how many weeks and days pregnant you are, measured from the first day of your last menstrual period. It's the standard clinical measurement — your midwife, sonographer and obstetrician all use it. Because conception happens around two weeks after LMP, gestational age is roughly two weeks longer than the actual age of the embryo.
How is "9w 3d" different from "9 months"?
Gestational age is measured in weeks and days because months have different lengths (28–31 days) and pregnancy decisions need daily precision. A typical pregnancy is 40 weeks — slightly more than 9 months, closer to 9 months and one week. Doctors and sonographers always use weeks + days, never months, because clinical windows are measured in days.
My LMP date is uncertain. How reliable is the result?
If you can't pinpoint the first day of your last period, LMP-based calculation becomes unreliable. The same applies if your cycles are irregular. In either case, a dating ultrasound scan between 7 and 13 weeks is the only accurate way to establish gestational age — it measures your baby directly.
Why is the calculator saying I'm more pregnant than I expected?
Because gestational age counts from your last period rather than from conception, most people are usually "two weeks more pregnant" than they feel. If you got a positive test around the time your period was due, you were already about 4 weeks pregnant by clinical measurement — not 2 weeks.
Which is more accurate — the calculator or my dating scan?
The scan, once it's been done. A dating scan measures your baby's crown-rump length and calculates gestational age directly from that measurement, accurate to within three to five days between weeks 7 and 13. Before the scan, the calculator is your best estimate. After it, the scan takes precedence if the two disagree by more than five days.
Does this calculator work for twin or multiple pregnancies?
Yes — gestational age is identical for singletons and multiples, since it measures time elapsed rather than the baby itself. However, twin pregnancies usually deliver earlier (typically 36–37 weeks for dichorionic twins), so the "weeks to go" figure should be interpreted with that in mind.
Book a dating scan at IUS London
A dating scan between 7 and 13 weeks measures your baby directly — the clinical gold standard for gestational age. Same-week appointments available in Kensington.