To some extent, you can use your period to estimate your baby’s due date. Most babies are born around 38 weeks after conception. Most people ovulate and conceive two weeks after their period, which is why you often hear people saying that pregnancy lasts for 40 weeks. If you have a regular 28-day cycle, you can roughly estimate your baby’s due day by adding 40 weeks to the first day of your last period.  However, this guess may be inaccurate if your cycle is irregular or longer or shorter than 28 days, you weren’t tracking your period, or you became pregnant while taking contraception that impacts your cycle, such as the pill. A dating ultrasound is significantly more accurate than using your period to determine your due date. Further to this, your foetal dating scan also looks for multiple pregnancies and significant anomalies.