Naegele's rule is one of the most common ways of calculating the expected date of delivery. Naegele's rule assuming that the pregnancy lasts 280 days. To calculate the expected date of delivery, add 7 days to the first day of the last menstrual period, subtracting 3 months, and add 1 year. In this case:

9 (day) + 7 (day) = 16 (day)

9 (month = September) - 3 (month) = 6 (month = June)

2018 (year) + 1 (year) = 2019 (year)

So, the estimated date of delivery is June 16th, 2019.