Calculate your pregnancy due date based on your last menstrual period. See your estimated due date, current pregnancy week, and trimester information.
Your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. This method assumes a 28-day cycle with ovulation on day 14. Most pregnancies last between 38 and 42 weeks.
First ultrasound scan (dating scan) is typically offered between weeks 8-14. The anomaly scan to check baby's development is offered between weeks 18-21. Regular antenatal appointments monitor both mother and baby throughout pregnancy.
Contact your midwife or maternity unit immediately if you have severe abdominal pain, bleeding, reduced baby movements after 24 weeks, severe headaches with vision problems, or if you think your waters have broken. The NHS provides excellent pregnancy care throughout the UK.
The due date is an estimate, not a deadline. Pregnancy length varies naturally between individuals based on cycle length, genetics, ethnicity, and whether it's a first pregnancy. While 280 days (40 weeks) from last menstrual period is the standard calculation, normal pregnancy duration ranges from 37-42 weeks. First-time mothers typically deliver around 40 weeks 5 days on average, whilst second and subsequent babies often arrive earlier around 40 weeks 3 days. Longer menstrual cycles (over 28 days) correlate with slightly longer pregnancies, whilst shorter cycles may result in earlier delivery. Ethnicity plays a role: South Asian women tend to deliver slightly earlier on average than white European women. The baby's genetics influence gestation length — if you were born early or late, your baby might follow similar patterns. Stress, nutrition, maternal age, and health conditions also affect delivery timing. Ultrasound dating in the first 12 weeks is considered more accurate than LMP dating as it directly measures the baby regardless of cycle irregularities, but even scan dates have 3-5 day margins of error. Rather than fixating on the exact due date, think of it as a rough guideline for planning maternity leave and hospital bags, with the understanding that "due month" is more realistic than "due date."
Going beyond 40 weeks is common and usually not concerning, but the NHS monitors you more closely after 41 weeks. Between 40-42 weeks, you'll have increased monitoring including membrane sweeps offered from 40-41 weeks to encourage natural labour. These involve a midwife performing an internal examination to separate membranes from the cervix, releasing hormones that may trigger labour naturally. Sweeps are uncomfortable but can be effective. By 41 weeks, you'll have additional monitoring appointments checking baby's movements, growth, and amniotic fluid levels. Induction of labour is typically offered between 41-42 weeks as risks increase beyond 42 weeks including stillbirth (though absolute risk remains low), placental insufficiency reducing oxygen and nutrients to baby, and meconium aspiration where baby passes first stool in utero. Induction methods include pessaries or gels containing prostaglandins to ripen the cervix, artificial rupture of membranes (breaking waters), or synthetic oxytocin drip to stimulate contractions. Some women decline induction preferring to wait for spontaneous labour, accepting closer monitoring. The decision balances risks of continuing pregnancy versus intervention. Your maternity team discusses options and supports your informed choice. Going to 41 weeks is very common; true post-term pregnancy (42+ weeks) is less common and does warrant intervention in most cases.
Early ultrasound dating (before 14 weeks) is more accurate than LMP-based calculation for most women. The 12-week dating scan measures crown-rump length (head to bottom) which correlates very precisely with gestational age in early pregnancy before individual growth variations emerge. This scan is accurate to within 3-5 days. In contrast, LMP dating assumes a regular 28-day cycle with ovulation on day 14, which doesn't hold true for many women. If you have irregular cycles, long cycles, recently stopped contraception, or conceived whilst breastfeeding, LMP dating can be significantly off. Some women don't track periods accurately or experience implantation bleeding mistaken for a light period, further complicating LMP calculations. If dating scan differs from LMP calculation by more than 5 days, the NHS uses scan date as the official due date because it's considered more reliable. Later ultrasounds (20-week anatomy scan) are less accurate for dating as babies grow at different rates by that stage, so they're used to check growth rather than establish dates. Women with very irregular cycles or those who don't remember LMP rely entirely on scan dating. Even with scan dating, remember the 3-5 day margin of error plus natural variation in pregnancy length means the "due date" remains an educated guess rather than precise prediction.
Last menstrual period: January 1, 2026. Due date: October 8, 2026 (280 days later). At May 1, 2026, you would be 17 weeks pregnant, in second trimester, with approximately 23 weeks (160 days) remaining. This standard calculation works well for women with regular cycles starting periods every 28 days consistently.
LMP: January 1, 2026. Standard calculation gives October 8 due date, but with 35-day cycle, you likely ovulated around day 21 (not day 14), meaning conception was a week later than standard calculation assumes. Actual due date might be closer to October 15, 2026. This is why early ultrasound dating is valuable — it would show the baby measuring a week smaller than LMP suggests, and the scan date would become the official due date.
If your cycles range from 25-40 days and you don't track ovulation, LMP dating is unreliable. In this case, the 12-week dating scan becomes essential. The scan might show the baby measuring 10 weeks 3 days when you thought you were 12 weeks based on LMP, giving you a new estimated due date roughly 10 days later than originally calculated. Always trust the early scan date in cases of cycle irregularity.
Register with a GP and midwife as soon as you know you're pregnant, ideally before 10 weeks to ensure timely booking appointment and early scans. The NHS provides free maternity care including antenatal appointments, scans, and delivery. Take folic acid (400 micrograms daily) from trying to conceive through first 12 weeks to reduce neural tube defect risk. Vitamin D supplementation (10 micrograms daily) is recommended throughout pregnancy and breastfeeding. Avoid alcohol completely as no safe level is established in pregnancy. Limit caffeine to 200mg daily (approximately two mugs of instant coffee). Avoid unpasteurized dairy, pâté, raw or undercooked meat, certain fish high in mercury, and unwashed salad to prevent food-borne infections. Start gentle exercise if not already active; continue existing exercise with modifications as pregnancy progresses. Plan maternity leave considering that babies may arrive up to 3 weeks early or 2 weeks late — most employers require notice and planning. Join antenatal classes (free NHS classes available) to learn about labour, pain relief, breastfeeding, and caring for newborns while meeting other expectant parents locally. Prepare your hospital bag by 36 weeks including comfortable clothes for labour, toiletries, baby clothes, nappies, and items for birth partner. Research infant car seats as you cannot legally leave hospital without one. Consider infant feeding choices and inform your midwife of your plans so they can provide appropriate support. If working, know your employment rights: maternity leave entitlements, statutory maternity pay, and protections against discrimination. Most importantly, attend all scheduled appointments and scans as they monitor both your health and baby's development throughout pregnancy.