A German doctor called Naegele came up with a formula to predict when a woman's child would be born. Although Naegele lived in the nineteenth century, many obstetricians still use this formula to predict the date of birth of a child.
Essentially, the formula is to take the date of your last normal menstrual period and add nine months and seven days (or 280 days). This is your due date.
Does it always work?
Every woman, and every pregnancy, is different. Naegele's rule is based on a "normal" 28-day menstrual cycle. If your cycle is longer or shorter than this, you should seek the advice of your doctor or caregiver for their predicted due date.
Be sure to incorporate regular ante-natal checks into your usual pregnancy routine.