Dr. Hunter, Professor of Obstetrics and Gynecology at McMaster University in Hamilton, Ontario, says that there is uncertainty about whether the baby's growth can be accurately predicted by a single ultrasound measurement. Furthermore, he says, "Not all small babies are constrained in growth and not all normal weight babies have been able to achieve optimal growth potential (Journal SOGC, Jan\91)".

Also, not enough is known about serial measurements of fetal growth and how they affect the outcome for the baby.

• To examine the placenta and see if it is located in a normal position.

Examination of the placenta is also essential for amniocentesis.

When there is bleeding in later pregnancy, ultrasound is used to locate the position of the placenta. The danger is that the placenta might be lying low in the uterus, blocking the baby's way out or partially separated from the uterine wall which can cause hemorrhaging. A low lying placenta is known as placenta previa and occurs in one in 200 pregnancies.

Early in pregnancy, the placenta often lies low down in the uterus, near the cervical opening. This is commonly reported on the US report as "placenta previa." This causes women considerable unnecessary anxiety and repeat ultrasounds. Actually, the position of the placenta at that early date is usually of no importance, since more than 90 percent of these low lying placentas move into a normal position by the time of birth.

Deciding On Whether To Have An Ultrasound

The most important thing is to be clear on the reasons you are having the ultrasound, and what information will be gained. You should also be clear on the benefits versus the risks.

Medical ethics specialists, Drs. Judith and Frank Chervenak and Dr. Lawrence McCullough, suggest a prenatal informed consent for US for every pregnant women. They believe the most ethical course is for full information to be provided to the woman including input from her doctor, and that the final decision be left up to her.

Some Questions You May Want To Ask Yourself

1. Is a complication suspected by your doctor?

2. Are you uncertain of the date of your last period?

3. Are you looking for malformations of the baby?

4. Would you consider aborting the baby if something showed up?

5. Is US required as part of another procedure such as amniocentesis or chorionic villi sampling?

6. What if the US showed an abnormality in the baby that wasn't actually present, as occasionally happens?

The Coalition for Medical Rights of Women in San Francisco recommends that women ask their doctors some or all of the following questions:

1. I'm not sure about the safety of ultrasound and I would like to wait a while. Is there any harm in that?

2. How will the information you get change my care? If it won't then why do it?

3. Are there other things you can do to find out the same thing and are they safer?

Some alternatives to ultrasound suggested by the Coalition include using a fetal stethoscope to hear the baby's heart beat (detectable around 18 to 20 weeks of pregnancy), having the position of the baby felt by an experienced doctor, and waiting until the next prenatal visit to assess the baby's growth.

Dr. Michael Klein, Research Director, Department of Family Medicine, McGill University, in Montreal, helps women in his practice clarify the issues around US. He asks his healthy pregnant women with no suspected complications and who also keep accurate records of their periods, how important is it to you to check for structural problems in the baby and would it make a difference for you to know? Otherwise, he tells them the US is medically unnecessary, but leaves the final decision up to them.