How is a due date calculated?
The standard method for calculating a due date is Naegele's Rule, named after the 19th-century obstetrician who popularised it. The formula is simple: add 280 days (40 weeks) to the first day of your last menstrual period. This is the method your midwife or GP will use at your booking appointment, and it's the same calculation this tool uses.
Naegele's Rule assumes a 28-day cycle and ovulation on day 14. If your cycle is longer or shorter, this calculator adjusts the estimate accordingly — adding or subtracting days to account for the difference from the 28-day baseline. This is why entering your actual cycle length gives a more personalised result than the standard formula alone.
It's important to understand what a due date actually is: it's the midpoint of a window of normal delivery, not a deadline. Roughly 80% of babies arrive within two weeks either side of the estimated due date. Full term is defined as 37 to 42 weeks — anything within this range is considered normal.
A note on accuracy: The LMP-based due date is an estimate. An early ultrasound scan — ideally between 10 and 14 weeks — provides more accurate dating by measuring the baby directly. If there's a discrepancy of more than 7–10 days between the LMP date and the scan date, healthcare providers typically use the scan date for ongoing care. Don't make major plans around a single date.
What do the three trimesters mean?
Your pregnancy is divided into three trimesters, each with distinct developmental milestones and physical experiences.
First trimester (weeks 1–12): This is the most critical period for development. All major organs form during these 12 weeks. The embryo becomes a fetus, heart activity begins, and limb buds form. It's also typically the most physically challenging period — nausea, fatigue, and breast tenderness are common. The risk of miscarriage is highest in the first trimester (around 10–20% of known pregnancies), which is why many people choose to wait until after the 12-week scan to share the news widely.
Second trimester (weeks 13–26): Often called the "golden trimester." For many people, nausea eases, energy returns, and the pregnancy becomes more visible. Around weeks 18–22, you may begin to feel the baby move (quickening). The 20-week anatomy scan gives a detailed look at the baby's development and is the standard screening for structural abnormalities.
Third trimester (weeks 27–40): Your baby gains most of its birth weight during these weeks. Discomfort increases — back pain, frequent urination, and difficulty sleeping are common. Your body is preparing for labour: your cervix softens and the baby typically moves into a head-down position. Antenatal appointments become more frequent as your due date approaches.
First steps after a positive pregnancy test
That second line. Take a breath. Here's a practical list of what to do in the days and weeks following a positive result:
- Start folic acid if you haven't already: 400mcg daily for at least the first 12 weeks reduces the risk of neural tube defects significantly
- Book with your GP or midwife: Do this as soon as possible to get on the antenatal care schedule. Most NHS areas aim for a booking appointment at 8–10 weeks
- Avoid alcohol and smoking: No amount of alcohol is considered safe in pregnancy
- Check your medications: Some prescribed and over-the-counter medications are not safe in pregnancy — review these with your GP
- Food safety basics: Avoid raw/undercooked meat and eggs, unpasteurised dairy, high-mercury fish, and raw sprouts. Wash all fruit and vegetables thoroughly
- Vitamin D: 10mcg daily is recommended throughout pregnancy in the UK
- You don't have to do anything immediately: Despite the cultural pressure to immediately announce, plan, or decide everything — you're allowed to sit with the news for a while
A note on accuracy — why ultrasound is still the gold standard
This calculator gives you an LMP-based estimate that is accurate to within a week for most people. But there are good reasons why a due date from an early ultrasound scan is preferred when there's any uncertainty.
Ultrasound dates the pregnancy by measuring the size of the embryo or fetus directly — specifically the crown-rump length (CRL) in the first trimester. This measurement is most accurate between 8 and 14 weeks. After 14 weeks, biological variation in fetal size makes scan dating less precise. This is why the 12-week scan is considered the primary dating scan.
If your LMP date suggests you're 8 weeks pregnant but your scan suggests 10 weeks, healthcare providers will typically update your dates to match the scan. This isn't unusual — it often happens when ovulation occurred earlier than the standard day 14 assumption, or when the LMP date is uncertain. Your care team will guide you through this.