How long pregnancy

by Dr. Lloyd Holm, D.O., Executive Director

Simple question – How long in months is the typical pregnancy?

If you answered 9, you’re not alone. If you answered 10, you are exactly correct. But how exactly is that? If you’ve ever wondered, “How can my doctor say I’m term at 40 weeks yet 9 months is 36 weeks’” you are not alone.

So much in life comes down to timing and pregnancy is no different.

Now, a little medical science to get us started. There’s a rule in obstetrics called Naegele’s Rule. It is used to determine a woman’s due date. It’s really quite simple. One takes the first day of the last menstrual period, adds 7 days, then subtracts 3 months. Therefore, if the first day of a last menstrual was December 5, adding 7 days would put one at December 12th, and subtracting 3 months would mean the due date would be September 12th.

But there’s a hitch in there. The dating begins at a time when the woman isn’t even pregnant! In fact, conception doesn’t typically occur until 14 days after ovulation which normally takes place 2 weeks AFTER the last menstrual flow.

But this fact only adds 2 weeks to the 9 months everyone assumes is the length of a pregnancy. Where do the other 2 weeks come from?

Again, it’s all about the timing.

The actual length of the human pregnancy is 281 days. And 281 days divided by 28 days (the length of time in a LUNAR month) is 10 months but the usually assumed length of pregnancy at 9 months goes by a CALENDAR month which averages 30 days.

Therefore, if you add 9 calendar months or 270 days to the 14 days from above, you have 284 days, a number close enough to 281 days or 10 lunar months to keep everyone happy.

Dr. Lloyd Holm is a licensed physician and OB/GYN. He started his career as a general practitioner in rural Iowa where he practiced for 7 years. Subsequent to this, he completed a residency in obstetrics and gynecology at the University of Nebraska Medical Center in Omaha. After practicing as a board certified OB/GYN for nearly 30 years, he retired in 2018. During his career he served as President of the Iowa State Board of Health, chairman of the College of Medicine Curriculum Committee at the University of Nebraska College of Medicine and OB/GYN Student Clerkship Director at the University of Nebraska College of Medicine. In addition, he is the author of two novels and a children’s book about the birth of Jesus. His writings have appeared in The Omaha World-Herald, Iowa Medicine, Obstetrics and Gynecology, the American Journal of Obstetrics and Gynecology, The Female Patient, and the medical website, Doximity. As the Executive Director of Options for Women/River Falls, Dr. Holm has continued his commitment to women’s well-being and is a steadfast supporter of its ministry and mission.


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