There are several simple methods used to determine a pregnancy due date with a calendar. No matter whether the due date is calculated by a high-tech web pregnancy due date calendar or from that odd circular slide rule the doctors and midwives use, the due date arrives from the same ancient knowledge and mathematical formula.
Ancient Due Dates
Our ancestors knew that if a sexually-active woman stopped having menstrual periods, she was probably pregnant and could expect a baby to be born about the time her tenth missed period was due. How did they know this before calendars were available? Easy. Our ancestors always noticed the phase of the moon. The moon was the only light at night and was the primary guide for religious events and planting seasons. The woman noted the phase of the moon as soon as she suspected she was pregnant. She would expect to give birth sometime close to the tenth time she sighted the moon in that phase. If she lived in a latitude which has four distinct seasons, she would expect her baby to be born at the end of the third season. For example, if she became pregnant in winter, she would expect a baby in the autumn.
The seasonal method is a rough guide, but the lunar phase method is extremely accurate. There are several reasons for the lunar phase method works as a means of calculating a pregnancy due date calendar:
- Most women have an average 28-day menstrual cycle.
- The moon has a 28-day cycle.
- Many women's cycles correlate with the phase of the moon, and they menstruate, or bleed, during the full moon. This correlation is strongest in areas without artificial lighting, but it is still common today.
- An average pregnancy lasts 280 days. Two hundred and eighty days is the same number as ten lunar months, or ten phases of the 28-day moon, or forty weeks, which equals nine and a half calendar months.
The Length of a Pregnancy Due Date Calendar
The folk wisdom that the normal duration of pregnancy is ten lunar months was confirmed by Dr. Naegele in the early 1800s. He observed thousands of "healthy peasant women" and confirmed that the average time between when their last normal menstrual period and when they went into labor was 37 to 42 weeks. The date of the last menstrual period is calculated from the day the bleeding begins. This range averaged out to 40 weeks and thus confirmed the old "ten moons" number.
But we don't use moons or lunar months to calculate dates anymore. Our modern calendars are based on a solar year of twelve months, with the months varying from 28 days to 31 days. We think of our months as lasting four weeks, but they are actually closer to lasting four and a half weeks long.
To calculate a modern due date we need to compensate for the fact that our months are longer than four weeks. We have to physically count 40 weeks on our calendars or use a device to count them for us, such as the circular "slide rule" device your midwife uses. Or we need to use a mathematical formula which adjusts for the difference. Fortunately Dr. Naegele did the math almost two hundred years ago!
Naegele's Rule
Naegele's formula allows us to quickly calculate a pregnancy due date calendar without gadgets or paperwork. Naegele's Rule says that forty weeks from any date on the calendar is equal to counting forward nine calendar months plus one week. So if we add nine months plus one week to the date which marks the first day of woman's last menstrual period, the result will be forty weeks, or the average "due date".
Using Naegele's Rule to Calculate a Date
If the first day of the last menstrual period is June 1, 2006, counting forward nine months brings us to March 1, 2007. Adding seven days gives us a due date of March 8, 2007.
We can use two handy shortcuts. Counting forward 9 months is the same as adding one year and subtracting 3 months. Nine equals 12 minus 3, right?) Therefore we can go back three months and then change the year. It is also easier if we change the dates into numbers. Now it is simple to calculate the date. The last menstrual period was 6/1/2006. We subtract 3 months and add 7 days to get 3/8/2006. And we remember to change the year, so the estimated due date is March 8, 2007.
Or, you can also use our handy calculator!
