Ultrasound can tell you whether the baby is still alive or whether a pregnancy is likely to continue after a threatened miscarriage. It can pick up about 90 percent of tubal pregnancies, if combined with vaginal ultrasound.
In the first three months of pregnancy, ultrasound is also necessary to perform special procedures like chorionic villi sampling or placing stitches in the cervix.
Except for the above reasons, ultrasound should not be performed in the first three months of pregnancy. If there were any side effects of ultrasound, the baby would be most susceptible at that time.
Dr. Douglas Wilson, Head of Prenatal Diagnosis at Grace Hospital in Vancouver puts it this way; "There's very little room for first trimester ultrasound. A lot of patients come in for early dating scans, but there's not much use in that. If someone's having bleeding or a complication, it's reasonable. But in the majority of cases, it's not necessary (Family Practice, Nov\89)."
Ultrasound is also used between 15 and 18 weeks pregnancy to date the pregnancy when the dates of the last menstrual period are unknown. At that time, it can predict the date of delivery to within two weeks in 89 percent of pregnancies.
The need for this particular use of ultrasound could usually be eliminated if all women of child-bearing years made it a practice to keep accurate records of their periods. Using menstrual history, the due date can reliably be predicted in between 75 and 85 percent of cases.
In high risk pregnancies, such as suspected twins, repeat Cesarean section, abnormal bleeding, diabetes or high blood pressure, elevated alpha fetal protein in the mother's blood (which may indicate a fetal abnormality) or pregnancies where amniocentesis or chorionic villi sampling is required as well as other special situations, the benefits of ultrasound far outweigh the possibility, as yet unproven, of long-term side effects.
Ultrasound is also useful if your doctor thinks you have too much waters or too little waters or if the baby is suspected to be in a position other than the usual head first one.
Dr. Ants Toi, Associate Professor of Radiology at the University of Toronto, says, "High fluid, I have not found to be as much a problem as low fluid, which is very serious, especially in early pregnancy." (SOGC Bulletin, July\Aug\87) Low fluid is associated with slow growth and fetal abnormalities.
The main type of abnormalities that can be detected are structural defects. This means that there are problems or defects as the baby forms. Amniocentesis, on the other hand, detects chromosomal or genetic problems.
Examples of structural defects are when the skull bone doesn't form over the brain, or the spinal bone only partially forms over the spinal chord, or the intestines form outside the abdomen, or one kidney is missing. After 20 weeks, US can also detect heart problems fairly accurately.
Ultrasound is also very useful for parents who have had a baby with this type of problem in the past and want to see if the problem has repeated itself with this pregnancy.
Successful detection of fetal abnormalities depends on the quality of the equipment, the expertise of the examiner and the reason the exam is done (high risk or normal screening). Not all abnormalities are detected, and there are false positives (ultrasound shows abnormality but the baby is normal) as well as false negatives (ultrasound normal, but baby has abnormality).
Dr. Toi estimates that about 50 to 75 percent of all possible abnormalities can be detected through ultrasound.
However, if the first US has not be done before 20 weeks, a single measurement at 32 to 36 weeks can be "notoriously inaccurate," according to Dr. James Youngblood at the University of Missouri, Kansas City of Medicine. (Journal of Family Practice, 1989).
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