Pregnancy Due Date Calculator: When Is Your Baby Due? Week-by-Week Guide

I still remember the moment my sister called me, barely able to get the words out. "I'm pregnant." After the screaming and the crying (happy tears, obviously), her very next question was: "So when is the baby actually due?" She'd taken the test that morning and was already mentally rearranging her entire calendar.

It's one of the first things every expectant parent wants to know, and it's surprisingly tricky to pin down. Only about 5% of babies arrive on their actual due date. But having that estimated date gives you a timeline to plan around — and that's incredibly valuable when your entire life is about to change.

Our Pregnancy Due Date Calculator works it out instantly based on your last period or conception date. But understanding how that date is calculated — and why it's really more of an estimate — is worth knowing.

How Is Your Due Date Calculated?

The most common method is called Naegele's Rule, and it's been used since the early 1800s:

Due Date = First day of your last menstrual period (LMP) + 280 days (40 weeks)

This assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter, the calculation adjusts accordingly — which is something our calculator handles automatically.

It's worth noting that pregnancy is technically counted from the first day of your last period, not from conception. This means that during "week 1" and "week 2" of pregnancy, you weren't actually pregnant yet. Confusing? Absolutely. But it's the standard used by the NHS and healthcare providers worldwide.

Alternative Calculation Methods

If you know your exact conception date (common with IVF or if you were tracking ovulation), the calculation is simpler:

Due Date = Conception date + 266 days (38 weeks)

For IVF pregnancies, the due date is calculated from the embryo transfer date, adjusted for the age of the embryo (3-day or 5-day transfer). Our calculator supports IVF date calculations as well.

How Many Weeks Pregnant Am I?

This is one of the most searched pregnancy questions, and the answer depends on when your last period started. If your last period began 8 weeks ago, you're approximately 8 weeks pregnant — even though conception likely happened around 6 weeks ago.

Pregnancy is divided into three trimesters, each with distinct developments and experiences:

First Trimester (Weeks 1-12)

The first trimester is when the most dramatic development happens, even though you might not look pregnant at all. Here's what's going on:

The first trimester is often the hardest emotionally. You're dealing with massive hormonal changes, possible morning sickness (which, despite the name, can strike at any time), and the anxiety of early pregnancy — all while potentially keeping it secret from most people.

Second Trimester (Weeks 13-26)

Often called the "golden trimester" because many women feel their best during this period:

The 20-week anatomy scan is a major milestone — it's when you can usually find out the sex (if you want to know) and when the sonographer checks that everything is developing normally.

Third Trimester (Weeks 27-40)

The home stretch. Baby is growing rapidly and you're definitely feeling it:

When Will Baby Actually Arrive?

Here's the reality that nobody tells you: your due date is really more of a "due month." Only about 5% of babies arrive on their exact due date. Most arrive within a two-week window either side:

• Before 37 weeks: Preterm (about 8% of births)
• 37-38 weeks: Early term
• 39-40 weeks: Full term (ideal)
• 41 weeks: Late term
• 42+ weeks: Post-term (induction usually offered)

First babies tend to arrive later than subsequent ones. The average first baby arrives at 40 weeks and 5 days. Second and third babies often arrive a few days earlier.

NHS Pregnancy Care Timeline

The NHS provides a structured schedule of appointments and scans throughout pregnancy:

• 8-12 weeks: Booking appointment with midwife
• 11-14 weeks: Dating scan (confirms due date and checks for Down's syndrome)
• 16 weeks: Antenatal appointment
• 18-21 weeks: Anatomy scan (detailed check of baby's development)
• 25 weeks: Antenatal appointment (first pregnancy only)
• 28 weeks: Antenatal appointment, blood tests
• 31 weeks: Antenatal appointment (first pregnancy only)
• 34 weeks: Antenatal appointment
• 36 weeks: Antenatal appointment, baby's position checked
• 38 weeks: Antenatal appointment
• 40 weeks: Antenatal appointment, discuss options if baby is late
• 41 weeks: Membrane sweep offered, induction discussed

The dating scan at 11-14 weeks is particularly important because it often adjusts your due date. The scan measurement is more accurate than calculating from your last period, especially if your cycles are irregular.

Early Pregnancy Symptoms

Before you even take a test, your body may already be dropping hints. Here are the most common early pregnancy symptoms, roughly in order of when they typically appear:

• Missed period: The classic sign, though some women experience light bleeding (implantation bleeding) around the time their period would be due
• Fatigue: Overwhelming tiredness is often one of the earliest symptoms, caused by rising progesterone levels
• Nausea: "Morning sickness" affects about 80% of pregnant women, usually starting around week 6
• Breast tenderness: Hormonal changes can make breasts feel sore, heavy, or tingly
• Frequent urination: Your kidneys start processing more fluid almost immediately
• Food aversions or cravings: Suddenly can't stand the smell of coffee? That's a common early sign
• Mood swings: Hormonal surges can make emotions feel more intense

If you're experiencing these symptoms and wondering whether you might be pregnant, a home pregnancy test is reliable from the first day of your missed period. Clearblue and other early detection tests can sometimes give results a few days before your period is due, though accuracy improves the longer you wait.

Conception Date Calculator

If you know your due date but want to work backwards to figure out when conception likely occurred, the maths is simple:

Estimated Conception Date = Due Date - 266 days

Or roughly: your due date minus 38 weeks. This gives you an approximate two-week window during which conception most likely happened.

Our Pregnancy Calculator can work both ways — enter your last period to get your due date, or enter your due date to estimate conception.

Ovulation and Fertility

Understanding ovulation is crucial both for getting pregnant and for calculating dates accurately. Ovulation typically occurs 14 days before your next period — not 14 days after your last one. This distinction matters if your cycle isn't exactly 28 days.

For a 28-day cycle, ovulation is around day 14. For a 32-day cycle, it's around day 18. For a 24-day cycle, it's around day 10.

You're most fertile in the five days before ovulation and the day of ovulation itself. Sperm can survive in the reproductive tract for up to five days, while an egg is viable for only 12-24 hours after release.

IVF Due Date Calculation

IVF pregnancies have more precise dating because the exact date of embryo transfer is known. The calculation depends on the type of transfer:

• 3-day embryo transfer: Due Date = Transfer Date + 263 days
• 5-day embryo transfer (blastocyst): Due Date = Transfer Date + 261 days

Some IVF clinics calculate from the egg retrieval date instead, adding 266 days. Our calculator supports IVF date calculations for both transfer types.

Things to Do Once You Know Your Due Date

Once you've calculated your due date, here's a practical timeline of things to organise:

First Trimester

• Book your first midwife appointment (call your GP surgery)
• Start taking folic acid (400mcg daily) if you haven't already
• Check which foods to avoid (raw fish, soft cheese, etc.)
• Look into maternity leave policies at work
• Consider when to share the news (many wait until after the 12-week scan)

Second Trimester

• Start thinking about birth preferences
• Research antenatal classes (NHS ones are free)
• Begin planning the nursery
• Look into childcare options (waiting lists can be long)
• Start a baby budget — use our Savings Calculator to plan

Third Trimester

• Pack your hospital bag (by week 36)
• Install the car seat
• Finalise your birth plan
• Stock up on essentials (nappies, wipes, muslins)
• Batch cook meals for after the birth
• Finish any work handover documents

Pregnancy Myths Worth Ignoring

"You're eating for two"

You really aren't. The NHS recommends no extra calories during the first two trimesters, and only an additional 200 calories per day in the third trimester. That's roughly one banana and a slice of toast — not a second dinner.

"The shape of your bump predicts the sex"

Carrying high, carrying low, carrying wide — none of it has any correlation with whether you're having a boy or a girl. Bump shape is determined by your muscle tone, body shape, and the baby's position.

"You can't exercise during pregnancy"

Unless your doctor specifically advises against it, moderate exercise during pregnancy is not only safe but beneficial. Walking, swimming, yoga, and light strength training are all excellent options. The NHS recommends 150 minutes of moderate activity per week throughout pregnancy.

"Morning sickness means a healthy pregnancy"

While nausea is common and generally not harmful, its presence or absence doesn't indicate anything about the health of your pregnancy. Some women have terrible morning sickness with perfectly healthy pregnancies, and others have none at all.

When to Contact Your Midwife or GP

While most pregnancy symptoms are normal, some require immediate attention:

• Vaginal bleeding (especially heavy or with pain)
• Severe abdominal pain
• Sudden swelling of face, hands, or feet
• Severe headaches that won't go away
• Vision changes (blurring, flashing lights)
• Reduced baby movements (after week 24)
• Fluid leaking from the vagina
• Fever above 37.5°C

If in doubt, always call. Midwives would much rather hear from you unnecessarily than have you worry at home about something that needs attention.

Related CalcTechLab Tools

• BMI Calculator — Check your pre-pregnancy BMI (not reliable during pregnancy)
• Calorie Calculator — Understand your nutritional needs
• Age Calculator — Work out exact ages and date differences
• Date Calculator — Calculate days between dates
• Savings Calculator — Plan financially for your new arrival
• Browse All Calculators — Explore our complete toolkit

The Bottom Line

Your due date is one of the most exciting dates you'll ever calculate. It gives you a target to plan around, a timeline to follow, and a countdown to the biggest day of your life.

But remember — it's an estimate. Babies have their own schedules, and yours will arrive when they're ready. Use our Pregnancy Due Date Calculator to get your date, follow the NHS care timeline, and try to enjoy the journey. It goes faster than you think.

My sister's baby, by the way? Arrived at 40 weeks and 3 days. Almost exactly on time. She was one of the lucky 5%.