One of the first things a mother-to-be wants to know is when to expect her little bundle of joy. Knowing this date will set the schedule for all the preparations and celebrations that the mother, friends and family want to do to welcome baby into the world. The future mother does not have to wait until her first doctor’s visit to find out this information. Her expected date of delivery is easy to figure out. She can simply calculate the date herself at home if she knows the first day of her last normal menstrual period.

With this information on hand, the future mother just has to add nine months and seven days to the first day of the last normal menstrual period and she will get her estimated date of delivery. This works out to 280 days in total, or 40 weeks, which is the average for a normal pregnancy.

So, for example, if the first day of a woman’s last normal menstrual period is June 1st, she would add nine months, which would mean March and then seven more days, to March 8. This means her expected date of delivery is March 8 of the next year.

Of course, it is wise to remember that most women do not give birth on their estimated date of delivery. A pregnancy is considered full term between 38 and 42 weeks, that is 10 days before to ten days after the estimated delivery date. Most women, about 80 percent, give birth during this time frame, though usually not on the exact date they estimated.

Some women give birth earlier, from week 20 to week 37, to premature babies, due to a variety of health complications, usually related to problems with the placenta, tobacco high blood pressure, diabetes, kidney problems, untreated thyroid gland issues, high fever and infections or other severe illnesses in the mother. Multiple-births (twins, triplets, etc.) also tend to be born earlier, even without any health complications.

Other women give birth after 42 weeks. There has not been a cause established for post-term births. However, it is believed that with most post-term births, there might have been a problem with the initial calculation of estimated delivery date, perhaps based on a wrong first day of last normal menstrual period, which means many of these babies might not be post-term after all. There are some risks related to post-term babies, which include the deterioration of the placenta and its ability to continue providing the fetus with the nutrients and oxygen it needs. There’s also the chance that the baby will pass meconium, fecal matter, into the amniotic fluid and ingest it into its lungs during delivery, which can cause pneumonia. One post-term birth also increases the chances for later post-term births.

It is important to know the true estimated date of delivery both for health and practical matters. Tracking menstrual periods as well as any abnormalities is a good way to ensure that the estimated delivery date will be correct.