Pregnancy Due Date Calculator
Estimated due date (EDD) is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14, for longer or shorter cycles, the calculator adjusts accordingly. If you know your conception or IVF transfer date, those can be used directly. In Australian obstetric care, the EDD is typically confirmed by a dating ultrasound at 7–12 weeks gestation, which measures fetal crown-rump length (CRL), the Hadlock method used by most Australian hospitals is accurate to within ±5 days when performed before 12 weeks. Pregnancy is divided into three trimesters: first (weeks 1–12), second (weeks 13–27), and third (weeks 28–40). This calculator shows your EDD, current gestational age, trimester, and a week-by-week milestone timeline including the combined first trimester screen (11–13 weeks) and morphology ultrasound (18–20 weeks).
Frequently Asked Questions
How is my due date calculated?
The standard Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle with ovulation occurring around day 14. This is the method most healthcare providers use as a starting point. If your cycle is longer or shorter than 28 days, your provider adjusts the calculation accordingly, typically adding or subtracting the difference in days.
How accurate is the due date?
Only about 4% of babies are born on their exact due date. First babies statistically tend to arrive slightly after their due date, while subsequent pregnancies often come a little earlier. A due date calculated from your last menstrual period (LMP) is accurate to within about 5-7 days when confirmed by a first-trimester ultrasound. However, only about 75% of babies are born within 2 weeks of their due date. Think of it as a mid-point of a 4-week window of possibility, not a deadline.
What is my trimester?
Pregnancy is divided into three equal-ish phases of roughly 13 weeks each. The first trimester (weeks 1-13 from LMP, or weeks 1-11 post-conception) is when morning sickness peaks and all major organs form. The second trimester (weeks 14-27) is often the most comfortable - nausea subsides, energy returns, and foetal movements become noticeable. The third trimester (weeks 28 to birth) brings rapid brain development, final organ maturation, and the baby positions itself head-down in readiness. Each phase has specific prenatal screening tests: the first-trimester combined screening (blood test plus nuchal translucency scan at 11-13 weeks) and the second-trimester morphology scan (18-20 weeks) being the most important.
What if my cycles are irregular?
The standard Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your menstrual cycles are consistently longer - for example, 35 days - you likely ovulate later than day 14, meaning your actual conception occurs after the assumed date. This causes the LMP-based due date to be too early. The opposite applies for shorter cycles. Your doctor or midwife will typically adjust your due date based on findings from your first ultrasound (dating scan) between 8-14 weeks, which measures the foetus's crown-rump length and gives a more accurate gestational age than menstrual dates alone when cycles are irregular.
When should I have my first ultrasound?
Your first ultrasound, called the dating scan, is best performed between weeks 8 and 14 of pregnancy. At this early stage, all foetuses are at similar developmental sizes regardless of genetics, which makes the measurement highly accurate, typically within 5-7 days of your true gestational age. After about 14 weeks, foetal growth begins to vary more between individuals, so measurements become less precise for dating purposes. This first scan also confirms foetal heartbeat and detects multiple pregnancies.
What is conception date vs due date?
Conception date is the day fertilisation occurred - when an egg and sperm fuse, typically around ovulation (day 14 of a standard 28-day cycle). The due date, however, counts forward from the first day of your last menstrual period (LMP), which is approximately 2 weeks before conception actually happened. This means on the day you conceive, your pregnancy is already counted as 2 weeks along by standard gestational dating. The reason is practical: most women know the date of their last period but rarely know exactly when ovulation occurred. So pregnancy starts 2 weeks before the biological beginning, making the standard 40-week pregnancy length actually 38 weeks post-conception.