Due Date Calculator
This calculator works out your baby's estimated due date from the first day of your last menstrual period, using Naegele's rule of adding 280 days (40 weeks), adjusted for your average cycle length if it differs from the standard 28 days. Enter your LMP date and select a cycle length from 25 to 35 days, and it returns your estimated due date, your current week and day of pregnancy, and which trimester you are in, alongside a visual trimester timeline. A key dates table shows your LMP, estimated conception date, the end of your first and second trimesters, full term at 37 weeks, your 40-week due date, and the post-term mark at 42 weeks. A separate panel covers New Zealand specific milestones: your dating scan window (8 to 12 weeks), anatomy scan window (18 to 20 weeks), the date to give your employer written notice for parental leave, when paid parental leave can start, and the days remaining until your due date. A short summary line pulls the headline figures together. Use it to plan appointments, work leave and travel around key dates, but treat the result as an estimate: only about 4 to 5 percent of babies are born on their exact due date, and your midwife or doctor will refine it using ultrasound measurements.
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How It Works
This calculator adds 280 days (40 weeks) to the first day of your last period (Naegele's rule), adjusted for your cycle length. Conception is estimated at approximately day 14 of a 28-day cycle. A dating ultrasound at 8 to 12 weeks provides a more accurate estimate. Only about 4 to 5% of babies arrive on their exact due date.