While there are a few women out there who still fear sonograms and refuse them, there is no safer way of checking the status of your baby than harmless sound waves floating through your amniotic sac and silently bouncing back to a paddle to make a black and white image of your child. Seeing the little rhythmic thump of the heartbeat gives more reassurance than any blood test can provide.

A sonogram is not useful, however, until five to six weeks into a pregnancy (counting from the beginning of your last period.) Prior to that point, only a sac can be measured, but at least an ectopic pregnancy can be ruled out. 

Some sonogram milestones:

• 4-5 weeks — pregnancy sac visible
• 5-6 weeks — fetal pole visible
• 7 weeks — heartbeat visible*

Babies should grow about a millimeter a day during these early weeks. At about nine weeks, the baby’s crown-rump length is easily measurable.

The margin of error in sonogram measurements is about seven days in the first trimester. By the third trimester, the measurement can be as much as three weeks off.

*A personal note. I found out the hard way that sometimes a fetal heartbeat can be obscured or just not visible. After bleeding heavily in my third pregnancy at 9 weeks, they could not find a definitive baby shape or a heartbeat in a sonogram. Because my hCG levels were high, though, we waited a few more days and had a second sonogram done. There was baby, clearly defined with arms, even, and a steady heartbeat. What might have happened was just a fluke of the machine, or perhaps an empty sac that was also in my womb (most likely from a twin pregnancy where the twin did not progress) got in front of the healthy one. This very situation is the reason I require two sonograms before a D&C is scheduled.