Most patients in the US get at least a basic sonogram around 18-20 weeks, though it may surprise you to find out that presently most medical associations do not recommend a routine ultrasound in pregnancy. A level II ultrasound takes longer and is more extensive than a level I ultrasound, but what exactly I being done may change from one institution to the other.
Many doctors still debate whether it’s worthwhile for all pregnant patients to routinely have an ultrasound done. The question is not only during which time of the pregnancy it should be done, but there is also controversy what that ultrasound should be checking for and how successful routine ultrasounds are in the detection of fetal anomalies.
An ultrasound in pregnancy can be either done as a “routine” ultrasound, usually between 18-20 weeks or it is done for specific reasons which usually depend at the time of pregnancy. There are essentially three different types of ultrasounds, depending on how much information is required:
Limited Obstetrical ultrasound
The Limited Obstetrical ultrasound (also known as level 1 ultrasound) is performed to answer a specific, clinical question, when there is no time or need to do the more extensive basic sonogram. The Limited ultrasound often takes only a few minutes and can be done anytime in pregnancy. Early in pregnancy it is often done to make sure the fetus is inside the uterus and to check the fetal heart rate, later on in pregnancy it is often done to check:
Basic Obstetrical ultrasound
The Basic Obstetrical ultrasound (also known as level 2 ultrasound) is more extensive than the limited ultrasound, and it takes normally between 30 and 60 minutes. It is done to answer a whole set of pre-defined questions and it also looks for specific fetal malformations, which in general the limited ultrasound does not do. This is usually the “routine” ultrasound most patients have at 18-20 weeks, and it is done at that time because the fetal anatomy is seen best, though it can also be done at other times too.
There are no uniform opinions among specialist what exactly should be
done on the Basic Obstetrical ultrasound. The procedure is often
different from one ultrasound center to the other. In general, the
following examinations are done with most Basic Obstetrical ultrasounds:
Targeted Obstetrical ultrasound
The Targeted Obstetrical ultrasound is usually done to look at specific fetal parts that were found abnormal on the basic ultrasound. Many ultrasound centers feel that there is not that much difference between a basic ultrasound and a targeted ultrasound.
Don’t expect that 100% of all problems in the fetus are being detected. The ability to detect an abnormality in the fetus at an ultrasound exam depends on a number of factors, such as the size and position of the fetus, the amount of amniotic fluid around the fetus, the body habitus of the mother (whether she is overweight or slim), the type of equipment used, and, most importantly, the skill and experience of the person performing the sonogram. Some major problems (such as anencephaly) are quickly diagnosed, but others such as cleft palate are not.
How many anomalies can be detected on the ultrasound?
Two studies, one was done in the US, the other in Europe showed an ultrasound detection rate of fetal malformations from 35% to 56%, in other words up to 65% of malformations are NOT detected by ultrasound.
In addition, there are both false-positive and false-negative examinations. A false-positive examination is an examination that shows something is abnormal, when in fact it is normal, and a false-negative examination shows a normal fetus, when there is in fact an abnormality.
When an ultrasound is being ordered during pregnancy always ask how extensive of an examination it is, who will be doing it, and make sure you know what the findings were.