Pregnancy Calculators
Nine clinical calculators — due date, ovulation, scan timing, NIPT eligibility and more. Built by our sonographer team. Free, instant, and paired with a real scan when you need one.
Pregnancy calculators estimate your due date, gestational week, ovulation window, ideal scan timing and other key dates from the information you already have — your last period, cycle length, conception date, or an IVF transfer date. This page brings all of them together in one place, each one answering a single, specific question.
If you're reading this, you probably want an answer quickly. You may be early in pregnancy and trying to work out how far along you are. You may be wondering whether it's too soon to book a scan, or too late for NIPT. You may have an irregular cycle and not trust the number a period-tracking app gave you. Whatever brought you here, you're looking for clarity — not a lecture.
IUS London is a specialist private ultrasound clinic in Kensington, two minutes from Notting Hill Gate. Every calculator on this site was built by our sonographer team to pair with a clinical scan, not replace one. We'll give you an answer now, and we're here if you want to confirm it on the screen.
Which pregnancy calculator do you need?
Pick the question that matches yours.
When is my baby due?
Calculates your estimated due date from LMP, conception date, or IVF embryo transfer date.
Open calculatorWhen did I conceive?
Works backward from your due date or last period to estimate the date of conception.
Open calculatorHow many weeks pregnant am I?
Gives your current gestational week and day.
Open calculatorWhich trimester am I in?
Tells you which trimester you're in and when the next one begins.
Open calculatorWhich scan should I book, and when?
Enter your current gestation and see the ideal window for every pregnancy scan — from early viability at 6 weeks through to anomaly, gender and growth scans.
Open the scan timing toolAm I far enough along for NIPT?
Confirms whether you've reached the 10-week threshold for non-invasive prenatal testing.
Open calculatorAre my hCG levels rising normally?
Compares two hCG blood test readings and calculates the doubling time.
Open calculatorHow much weight should I gain?
Uses your pre-pregnancy BMI to give a recommended weight gain range based on NICE guidelines.
Open calculatorNot sure which scan you need?
The IUS Scan Advisor asks a few questions and recommends the right scan based on your gestation, symptoms, and reason for scanning. Under a minute, with a direct booking link.
What is a pregnancy calculator?
A pregnancy calculator applies a standard clinical formula to the dates or measurements you provide. The most common, Naegele's rule, adds 280 days to the first day of your last menstrual period to estimate your due date. Other calculators use conception date, embryo transfer date, ultrasound measurements, or hormone levels.
What a calculator does well
- Gives a reasonable first estimate of gestational age and due date
- Identifies the right window for booking a scan or NIPT
- Helps you plan around key pregnancy milestones
- Works instantly, any time, without a referral
What a calculator cannot do
- Replace an ultrasound measurement of the baby
- Account for irregular cycles or uncertain dates
- Confirm viability or detect complications
- Match the accuracy of a dating scan (±3–5 days)
A due date based on your last period is accurate to within about a week at best, and only when your cycles are regular. NICE guideline NG201 recommends ultrasound dating as the clinical standard.
How accurate are pregnancy calculator results?
Accuracy depends on what you put in and what the calculator is doing with it. The table below shows how each dating method compares — and why the earlier the scan, the tighter the window.
| Dating method | Accuracy window | Reliability | Notes |
|---|---|---|---|
| IVF embryo transfer date | ± 1 day | Highest | Conception date is known exactly |
| Dating scan (7–13 weeks) | ± 3–5 days | Very high | Measures crown-rump length directly; NICE-recommended standard |
| LMP — regular cycle | ± 7 days | Moderate | Assumes 28-day cycle, ovulation on day 14 |
| LMP — irregular cycle | ± 2+ weeks | Low | Estimate drifts when cycle length varies; scan recommended |
| Dating scan (after 14 weeks) | ± 1–2 weeks | Moderate | Babies vary more in size after 14 weeks; earlier is better |
Between weeks 7 and 13, a sonographer measures the crown-rump length and calculates gestational age directly from that measurement. This is the method both NICE and the Royal College of Obstetricians and Gynaecologists recommend for confirming dates.
When you need a scan, not a calculator
A calculator is a starting point. These situations call for a scan — and the sooner the better.
Both are available same-week at our Kensington clinic.
- Your cycles are irregular, unusually long, or unusually short
- You don't remember your last period confidently
- You conceived while on hormonal contraception
- You had IVF or assisted conception and want viability confirmed
- You've had bleeding, cramping, or pain in early pregnancy
- A previous scan was inconclusive and you're due a follow-up
- You're past your estimated due date and haven't had a recent review
Calculators built by a specialist London ultrasound clinic
IUS London is a private ultrasound clinic at 5a Lucerne Mews, W8 4ED, two minutes' walk from Notting Hill Gate station. We scan pregnant women from as early as six weeks, with same-week appointments and discreet, specialist care. Every calculator on this site was designed by our sonographer team, and every result links to a scan booking when clinical confirmation is the right next step.
Specialist sonographers
All scans are performed by HCPC-registered sonographers — not a general clinic. Many hold concurrent NHS positions.
Same-week appointments
Often available same day. No GP referral required. Book online in under two minutes.
Scans from 6 weeks
Among the earliest scan windows available in London. Viability confirmed, heartbeat checked.
Kensington, W8
Ground-floor clinic at 5a Lucerne Mews, two minutes from Notting Hill Gate. Fully accessible.
Results within 2–4 hours
Immediate verbal feedback during your scan. Written report delivered digitally within 2–4 hours.
CQC registered
Registered with the Care Quality Commission. The same clinical standards you'd expect from the NHS.
Frequently asked questions
Confirm your dates with an ultrasound scan
Calculators give you a starting point. A scan gives you certainty — and the first glimpse of your baby.