Due Date Calculator: When Will Your Baby Arrive?
Your due date is calculated as 280 days (40 weeks) from the first day of your last menstrual period — but only 5% of babies actually arrive on that exact date. Most pregnancies last anywhere from 266 to 294 days, which is why due date calculators give you a window, not a target. This guide explains how the calculation works, why babies don't follow a schedule, and how to plan your finances and maternity leave around that uncertainty.
How Due Dates Are Calculated
The standard method is called Naegele's Rule. You take the first day of your last menstrual period (LMP), add 280 days, and that's your estimated due date. Most NHS services and pregnancy apps use this formula automatically — you enter a date, they add 40 weeks, done.
If you know your conception date instead of your LMP, you add 266 days (since conception typically happens around day 14 of a 28-day cycle). Our due date calculator handles both methods. If your cycles are longer or shorter than 28 days, the calculation shifts — cycle length matters because ovulation timing varies between people.
Ultrasound dating (usually done between 8–14 weeks) is actually more accurate than LMP-based dating for predicting when labour will start, so if you have an early scan, your midwife may adjust your due date based on measurements. Don't be alarmed if it shifts by a few days — that's normal and expected.
Why Babies Don't Stick to the Schedule
Babies aren't particularly punctual. Here's the reality: only 5% arrive on their due date. About 80% arrive within two weeks either side of the estimated date. The remaining pregnancies go significantly longer or shorter.
Why babies are late: Longer pregnancies are slightly more common than shorter ones. First-time mothers often deliver a few days after their due date. Genetics play a role too — if your mother or sister had long pregnancies, you're more likely to as well.
Why babies come early: Infections, pre-eclampsia, gestational diabetes, multiple pregnancies, and certain medical conditions can trigger labour before 40 weeks. Some pregnancies simply run shorter naturally. Stress and strenuous activity don't reliably induce labour despite what folklore suggests.
What changes with age and health: Women over 40 are slightly more likely to deliver early. Those with pre-existing conditions (high blood pressure, diabetes) often deliver earlier than due date. Multiple pregnancies (twins, triplets) almost always deliver before 40 weeks — typically around 37 weeks for twins.
This variability is why maternity leave planning is trickier than it sounds. Your due date is really a "due window," and you should plan accordingly.
Planning Maternity Leave Around Uncertainty
In the UK, you can start maternity leave anytime from 11 weeks before your due date (the earliest date is called the "qualifying week"). Most people start 2–4 weeks before the due date, which gives you a buffer. If your baby arrives early, you're already on leave. If they arrive late, you have time to rest before labour.
Statutory maternity pay is 90% of your average weekly earnings (capped at £184.03/week) for the first 6 weeks, then £184.03/week for the next 33 weeks, totalling 39 weeks of paid leave. If you don't qualify for statutory pay, you may be entitled to Maternity Allowance instead. Many private employers offer enhanced packages — check your employee handbook or ask HR, because this directly affects how much you need to have saved before labour.
The key: don't assume you know your income for the 9–12 months after birth. Build a financial buffer of 3–6 months of household expenses before your due date. If your partner is taking Paternity Leave (2 weeks paid, at the same rates as maternity pay) or Shared Parental Leave, factor that into your household income planning too.
Budgeting for the First Year
The cost of raising a child in the UK from birth to age 18 is estimated at £150,000–£200,000, but that's a long-term figure. For the first year specifically, you're looking at significant upfront costs.
Baby essentials (first year): Budget £1,500–£3,000 for a pram, cot, car seat, steriliser, bottles, and clothing. This is one area where it's safe to buy second-hand for non-safety items (clothes, toys, some furniture), but invest new in car seats, mattresses, and anything that's had a crash or unknown history.
Nappies and feeding: Budget £100–£150/month for nappies alone. If bottle-feeding, add formula costs (£50–£100/month depending on brand and whether you need specialised formula).
Childcare: Once you return to work, childcare becomes the second-biggest expense after housing for many families. The average UK nursery costs £1,100–£1,400/month for full-time care. If you have multiple children in childcare simultaneously, the costs multiply, though most settings offer a discount for siblings. Use our childcare cost calculator to see what you're facing in your area — costs vary wildly by region and provider.
The 30 hours free childcare (for eligible 3–4 year olds) reduces costs significantly, but it only covers 30 hours/week of term-time hours, and doesn't start until age 3. If you're returning to work before then, you're covering full childcare costs. Some employers offer childcare vouchers or salary sacrifice schemes, which save you National Insurance contributions — ask HR if your workplace participates.
Our baby budget planner breaks down what's essential versus nice-to-have, and our cost breakdown for the first year gives detailed month-by-month guidance.
Preparing Physically and Medically
Your due date changes slightly when measured by ultrasound — earlier scans (around 12 weeks) are more accurate than LMP estimates. Some women have a longer or shorter cervix naturally, which can affect labour timing. Gestational diabetes, pre-eclampsia, and other pregnancy complications may mean your baby is safer arriving earlier, so your midwife may recommend induction or planned C-section before the due date.
Folic acid (400mcg daily from before conception through week 12) reduces neural tube defect risk by 70%. Vitamin D (10mcg daily) throughout pregnancy is recommended by the NHS. If blood tests show anaemia, iron supplements are important — iron deficiency in pregnancy increases fatigue and increases risk of complications.
Moderate exercise (150 minutes per week) throughout pregnancy is recommended, unless your midwife advises otherwise. Swimming, walking, and prenatal yoga are popular because they're low-impact. Avoid contact sports and anything with a high fall risk.
Your pre-pregnancy weight is relevant too. If you start at a healthy BMI (18.5–24.9), you should expect to gain 11.5–16kg over the full pregnancy. These guidelines vary by BMI — speak to your midwife about what's appropriate for you.
Multiple Pregnancies Change the Timeline
If you're expecting twins, your due date is calculated the same way — 40 weeks from LMP — but the statistical due date is earlier. Twin pregnancies average 36–37 weeks; triplets average 32–33 weeks. This isn't because the calculation is wrong; it's because multiple pregnancies carry more risk of complications (gestational diabetes, pre-eclampsia, early labour), so babies are safer delivered sooner. Your midwife will discuss delivery timing with you, and you'll likely have more frequent scans and monitoring than a single pregnancy. See our post on multiples and the fertility connection for more detail.
Frequently Asked Questions
Can I influence my due date by changing my lifestyle? No reliably. Walking, spicy food, sexual activity, and acupuncture are often recommended in folklore to induce labour, but evidence suggests they don't shift your due date by more than a day or two, if at all. Your baby arrives when their body is ready. If you're overdue (past 42 weeks), your midwife will discuss induction rather than waiting.
What if my due date is wrong? Early ultrasound (8–14 weeks) is more accurate than LMP-based dating. If your scan date differs from your LMP date by more than a few days, your midwife will update your due date to match the scan. This doesn't mean your original date was "wrong" — it means the ultrasound is more precise.
Should I stop working before my due date? No legal requirement, but most people stop 2–4 weeks before due date. You can work right up to labour if you feel able, but given that only 5% of babies arrive on time, many people find it less stressful to already be on leave if labour starts unexpectedly.
What if I go into labour before 37 weeks? That's considered "preterm labour." Speak to your midwife or labour ward immediately. Modern neonatal care means babies born after 32 weeks have very good survival and health outcomes, though they may spend time in hospital. Before 32 weeks, medical intervention to delay labour (if possible) is usually attempted.
How do I know if I'm in labour? Regular contractions that get closer together and more intense (every 3–5 minutes, lasting 60+ seconds), vaginal bleeding, or fluid leaking from the vagina are signs to call your midwife or labour ward. Braxton-Hicks (irregular tightenings) are common from around 20 weeks onwards and don't mean labour is imminent.
Should I induce labour if I'm overdue? Your midwife will discuss induction if you reach 42 weeks, because risks of stillbirth and complications increase. Before that, it's a conversation between you and your clinical team based on your health, the baby's health, and your preferences. Don't feel pressured to be induced at exactly due date — slight overdue pregnancies are common and usually safe, but your midwife will monitor closely.
Can I plan conception to hit a specific due date? You can estimate it — ovulation (your fertile window) happens roughly 14 days before the next period in a regular 28-day cycle. But cycle length and ovulation timing vary, so exact timing is hard to predict. Our fertility guide goes into more detail on conception timing and conception-based dating.
What if I miscarry before the due date? Miscarriage is more common than many people realise — roughly 1 in 5 pregnancies end in miscarriage, usually before 12 weeks. If you're concerned about spotting or cramping, contact your midwife or GP immediately. Many miscarriages cannot be prevented, but early medical assessment can rule out other causes and ensure you're physically safe.
Your due date is a useful anchor point for planning, not a guarantee. Most babies arrive within two weeks either side of the date — which is why maternity leave planning, childcare booking, and financial buffers should all assume some variability. Use our due date calculator to identify your window, cross-reference it with maternity pay entitlements, and build a 3–6 month financial buffer before labour. Start planning now, because babies — true to form — operate on their own schedule.