During Jessica Yager's pregnancy with her son, Garrett, she looked online trying to determine how her little one was developing, how big he was growing as each month ticked away. But different sites held conflicting information. Was her baby the size of a peanut or about an inch long? And why did sites offer such different facts?

"Part of what makes this so confusing is that when I say someone is 8 weeks pregnant, it's actually six weeks from conception," says Dr. Marjorie Greenfield, an associate professor in the Department of Obstetrics and Gynecology at the University Hospitals of Cleveland and the author of The Working Woman's Pregnancy Book (Yale University Press, 2008). "And what's even more confusing is that in books and on the Internet some places are using the actual embryonic weeks and some are using the obstetric weeks. That's why the sizes and developmental information provided can differ so much."

To avoid the confusion that Yager and countless other moms have experienced, we'll start from the beginning and use obstetric weeks.

"Basically on the day of conception, the cells begin to divide," Dr. Greenfield says. "For the first couple of weeks, the 'baby' is just a ball of cells, which moves from the fallopian tube (where fertilization occurred) down into the uterus, where it burrows into the wall of the uterus."

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