Pregnancy Due Date Calculator

The estimated due date is calculated by adding 280 days, or 40 weeks, to the first day of your last menstrual period. That is Nägele's rule: add 7 days to the LMP date, then add 9 calendar months, or equivalently subtract 3 months and add 1 year. An LMP of 1 May gives a due date of 5 February the following year. The method assumes a 28-day cycle with ovulation on day 14.

Cycle length shifts the answer. If your cycle runs longer than 28 days you add the extra days, and if it runs shorter you subtract them, so a 35-day cycle pushes the estimated date forward by 7 days. Where you know the conception date instead, the calculator adds 266 days, or 38 weeks, since that counts from fertilisation rather than from the start of the period.

The NHS offers every pregnant woman a dating scan between 8 and 14 weeks, which measures the baby's crown-rump length and refines the date. That scan overrides the LMP calculation when the two differ by 7 days or more, because ultrasound at this stage is the more accurate of the two. Later scans are poorer for dating, as growth rates diverge between individuals. Only about 4% of babies arrive on their exact due date and 80% are born within 2 weeks either side, with 37 weeks counting as full term, 41 as late term and 42 as post-term.

Antenatal care follows a set schedule. The booking appointment falls between weeks 8 and 12, covering full medical history, blood tests and the screening on offer. The dating scan and Down's screening come at 11 to 14 weeks and the anomaly scan at 18 to 21 weeks. A first pregnancy adds midwife checks at weeks 25, 31, 34 and 36. Week 28 brings blood tests and anti-D if you are Rh negative, and there are further checks at 38, 40 and 41 weeks, with induction discussed from 41. Twin, multiple and high-risk pregnancies get extra appointments throughout.

Maternity entitlements are keyed to the due date as well. Statutory Maternity Pay runs at 90% of weekly earnings for the first 6 weeks, then £194.32 a week or 90% of earnings, whichever is lower, for the remaining 33 weeks. That is 39 weeks paid within 52 weeks of leave, the last 13 unpaid. Eligibility needs 26 weeks of continuous work with the same employer by the fifteenth week before your due date, and you have to tell your employer at least 15 weeks before. Maternity Allowance covers the self-employed and anyone who does not qualify for SMP, at £194.32 a week or 90% of average earnings, whichever is lower, for up to 39 weeks. Pay can start 11 weeks before the due date at the earliest, when you are 29 weeks pregnant.

Trimesters divide the pregnancy into three familiar stretches. The first, weeks 1 to 12, brings nausea, fatigue and breast tenderness, and carries the highest miscarriage risk at 10 to 25% of confirmed pregnancies. The second, weeks 13 to 27, is usually the most comfortable, with movements felt between 16 and 22 weeks and the anomaly scan at 18 to 21. The third, from 28 weeks onwards, grows steadily less comfortable. Viability is reached at 24 weeks, which is also the legal limit for abortion in the UK, term begins at 37 weeks and overdue starts at 42.

Supplements come down to a short but well-evidenced list. The NHS recommends 400mcg of folic acid daily from 3 months before conception until week 12, which cuts neural tube defects such as spina bifida by 70%. A prescription-only 5mg dose applies if you have diabetes, a BMI of 30 or more, a family history of neural tube defects, or you take anticonvulsants. Vitamin D at 10mcg daily is advised throughout pregnancy and breastfeeding, a recommendation in place since 2016. Avoid liver and vitamin A supplements, which are teratogenic in high doses, along with unpasteurised dairy, undercooked meat, alcohol and smoking. Caffeine is capped at 200mg a day, about 2 mugs of coffee.