Scan Timing Calculator
Tell us how far along you are. We'll show you which pregnancy scans are available right now, which are coming up, and the exact clinical window for each one — with a direct booking link.
Work out your scan schedule
Choose the date you know. The calculator does the rest.
Cycle length adjustment (for irregular or long/short cycles)
Where you are in your pregnancy
Every scan, with its window and status
How this calculator works
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day of conception. This is the international clinical convention and it's what your midwife, sonographer and obstetrician will use. The calculator adds your LMP to today's date to work out how many days pregnant you are, then checks that against the accepted clinical window for each scan.
If you had IVF, the calculator uses your transfer date and the embryo age (day-3 or day-5) to derive the equivalent LMP. If you already know your gestation from a recent dating scan, the calculator trusts that directly — a dating scan is more accurate than any date-based calculation.
For irregular cycles, the cycle-length adjustment shifts your notional LMP to reflect later or earlier ovulation. This matters: a woman on a 35-day cycle ovulating on day 21 has a gestational age one week lower than the default calculation would suggest.
Why dates matter
Each scan has a clinical window — a range of gestational ages during which the scan is both technically possible and clinically meaningful. Outside that window, the scan may produce incomplete or unreliable results.
The nuchal translucency measurement, for example, is only valid between 11 weeks and 13 weeks 6 days. Outside that range, the fluid measurement is no longer a reliable Down's syndrome screening marker.
Knowing your exact gestation lets you book at the right time — not too early, not too late.
Scan windows at a glance
The reference table below shows the clinical window for every scan we offer. The calculator uses these exact thresholds.
| Scan | Earliest | Latest | Best window |
|---|---|---|---|
| Early Pregnancy Scan | 6w 0d | 10w 6d | 7–9 weeks |
| Dating Scan | 8w 0d | 14w 0d | 11–13 weeks |
| Nuchal Translucency Scan | 11w 0d | 13w 6d | 12 weeks |
| NIPT | 10w 0d | Any time after | 10–14 weeks |
| Gender Scan | 16w 0d | Any time after | 16–20 weeks |
| Anomaly Scan | 18w 0d | 22w 6d | 20–22 weeks |
| Growth & Wellbeing Scan | 24w 0d | Full term | 28, 32, 36 weeks |
| 3D/4D Bonding Scan | 24w 0d | 32w 0d | 26–30 weeks |
| Presentation Scan | 36w 0d | Full term | 36–38 weeks |
When to run this again
Re-run the calculator after your dating scan. A dating scan measures your baby directly and produces a more accurate gestational age than any date-based method.
Frequently asked questions
Why is pregnancy dated from the last period rather than conception?
Because most people can recall the first day of their last period, but few can pinpoint the exact day of conception. Dating from LMP is a reliable, reproducible convention used worldwide. It means gestational age is always about two weeks longer than actual embryo age — which is why a "six-week" pregnancy contains a four-week-old embryo.
My dating scan said I'm further along than the calculator does. Which is right?
The scan. A dating scan measures your baby's crown-rump length directly and is accurate to within three to five days between weeks 7 and 13. The calculator can only estimate from dates you provide. If the two disagree by more than five days, clinical practice is to re-date the pregnancy from the scan measurement.
Can I book a scan before its "earliest" date?
It's rarely a good idea. Each scan has a minimum gestation because below that, the structures the sonographer needs to see aren't yet visible or measurable. An anomaly scan done at 16 weeks, for example, can't reliably assess the heart. The windows are there to make the scan worthwhile.
What if I'm past the "latest" date for a scan?
Some scans have a hard cutoff for clinical reasons — the nuchal translucency measurement is only valid up to 13 weeks and 6 days, for example. Others, like the anomaly scan, are technically still possible after the window but less reliable. If you've missed a window, please call the clinic — we can often suggest an alternative that achieves the same clinical goal.
Does this calculator work for twin or multiple pregnancies?
The dating and gestational age maths are identical. However, the scan schedule for multiple pregnancies is more intensive — you'll need growth scans every two to four weeks from 20 weeks rather than the standard intervals. Please speak to our team directly so we can build a schedule around your specific pregnancy.
Book your next scan at IUS London
Same-week appointments at our specialist ultrasound clinic in Kensington. Or talk to our team if you'd like help choosing the right scan for your stage.