Obstetric ultrasound is undoubtedly the most powerful means of evaluating the fetus when performed by experienced, trained personnel*  Fetal "screening" may be performed as either part of a basic exam ("level 1") or a detailed scan ("level 2" or higher).  A basic or level 1 scan does not need to make a specific diagnosis but should be able to tell when something is wrong, resulting in a referral for a detailed or "level 2" scan.  Both the "screen" and the "diagnosis" can even take place at the time of a single ultrasound.

In practical terms, what ultrasound can determine will vary with gestational age and we can categorize the pregnancy into 3 trimesters. 


A first trimester ultrasound can provide very useful information. Of course, the ultrasound should determine the number of fetuses, the gestational age, and basic information about the uterus and ovaries/ adnexa. Later in the first trimester, ultrasound can also determine the location of the placenta. Common conditions ultrasound can determine include incorrect menstrual dating, nonviable gestations (even if you feel perfectly normal), multiple pregnancies including the placental type (usually dichorionic or monochorionic), subchorionic hemorrhage (usually, not always associated with vaginal bleeding), uterine fibroids, and ovarian masses.

In terms of fetal screening, a surprising number of major structural defects can be detected, especially at 10-14 weeks.  Also, first trimester ultrasound performed at 11-13 weeks can evaluate the risk of fetal aneuploidy, primarily by nuchal translucency measurement.


A carefully performed second trimester ultrasound can provide more information than any other single ultrasound. This information can be categorized into 4 parts:

  • Determine gestational age and confirm normal fetal growth.
  • Detect major fetal anomalies or, more commonly, confirm normal fetal development. A detailed fetal anatomic survey might be considered the "baby's first physical exam" because it is performed much like a physical exam, from "head to toe". It can actually detect some types of abnormalities that would not be suspected by the usual physical exam after birth, although many other birth defects- usually minor- will be detected by the physical exam after birth but not the ultrasound survey.  A carefully performed second trimester sonogram can detect the majority of important birth defects including defects associated with normal chromosomes which would not be detected by amniocentesis. For example, ultrasound can detect nearly all neural tube defects whereas AFP screening detects 80-90% of neural tube defects. Click here for a slide show of a normal fetal anatomic survey.
  • Evaluate "soft" or "minor" ultrasound findings that might indicate a higher risk of fetal aneuploidy, including Down syndrome. Second trimester ultrasound can look for a variety of subtle differences between normal fetuses and those with Down syndrome as part of a so-called “genetic sonogram”. A carefully performed genetic sonogram can detect 50-70% or perhaps even more of cases of fetal Down syndrome when performed at experienced centers. Click here for more information regarding a genetic sonogram. This information is best correlated with results of a biochemical screen. Click here for more detailed information in a powerpoint slide show explaining screening options for fetal Down syndrome using ultrasound and biochemistry.
  • Evaluate other important parts not directly related to the fetus: the placenta, fluid, cervix, uterus, and adnexa.


The third trimester is the best time for evaluating fetal growth, since fetuses add most of their weight during the second half of pregnancy. Third trimester ultrasound can also evaluate amniotic fluid, blood flow through Doppler studies, the cervix, placenta, and fetal activity. It is not a good time for fetal screening but is an important time for following and clarifying suspected abnormalities. 3D/ 4D ultrasound performed in the early third trimester usually provide images that look more like a "baby" and less like a fetus because soft tissues are beginning to fill out. 3D/4D ultrasound is still useful at other times during the pregnancy, but these images may not be as easily appreciated by the parents.


Obstetric ultrasound is undoubtedly the most powerful method of fetal screening. At the same time, it's power may be limited and even dangerous in the hands of inexperienced or untrained personnel. The value of ultrasound reflects primarily the training of the personnel and much less the quality of the machine. There are several ways to determine whether your ultrasound is being performed by qualified personnel. If the exam is performed by a sonographer, the sonographer should be RDMS certified and ideally should have a subspeciality certificate of competence in Obstetrics. It is more difficult to determine whether the interpreting physician is qualified since, believe it or not, there is no uniform certification for physicians and they may come from different fields- obstetrics, radiology, or even genetics. In general, the more time the physician spends doing ultrasound for a living, the better that person probably is. Time spent delivering babies or performing other types of radiology exams detracts from expertise in ultrasound.