Ask the Midwife Kathleen McClelland has been a midwife since 1990. Prior to that she was a Family Nurse Practitioner, with an emphasis on Women's Health Care, working mainly with the Deaf and Native American populations. As a midwife she has worked in large and small health care settings, from a a large health maintenance organization to a small private practice in a rural community to a medium size hospital caring for indigent women. She has attended one home birth, as the birth assistant to the midwife, of a very dear friend. The birth of her son was at a free-standing birth center in Alexandria, Virginia, attended by two of her midwifery classmates as well as her husband, sister, and father.		Midwife Kathleen McNellis Ask Kathleen a question

January 10 --

Q: Dear Kathleen:
According to my last menstrual period dates I was 20 weeks and 5 days pregnant when I had a sonogram done last week. However, the sonogram determined that the baby measured only 19 weeks 2 days gestation. Is this difference something I should be worried about? Is my baby too small? Is my due date different from what we thought?

Thanks,
Sonja

Kathleen replies:

A: Dear Sonja,

Of course, you (as well as all pregnant women) are anxious to know the "due date" of your baby. A "due date" determined by a middle of the second trimester sonogram, such as yours done at 20 weeks and 2 days, is accurate to within 7-10 days. So you see, given the range of accuracy of a second trimester sonogram, this matches up precisely with the "due date" determined by your last menstrual period. Be sure to discuss this with your midwife or doctor, but I would expect that your "due date" will be the one that extends 280 days from the first day of your last normal period, especially if other factors in determining your due date (such as the size of your growing uterus and the day of quickening) support this date.

With rare exceptions, I like to think that your baby's actual "due date" is your baby's birthday, considering spontaneous labor. And remember, the "due date" is just an estimate or an educated guess. Sometimes, women are very in touch with their monthly cycles and are therefore certain which day they conceived. This information should be shared with your midwife or doctor. It is generally accepted that the average length of pregnancy is 266 days from conception or 280 days from the first day of the last menstrual period, based on an average menstrual cycle of 28 days. Of 100 women, only 5 will birth their babies on their actual "due date". The remainder will birth their babies up to 2 weeks before or after this date. All will be considered full-term pregnancies.

I hope this information is helpful, Sonja.

Sincerely,

Kathleen McClelland, CNM

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