In practical terms, what ultrasound can determine will vary with gestational age and we can categorize the pregnancy into 3 trimesters.
FIRST TRIMESTER (5-14 WEEKS)
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.
SECOND TRIMESTER (14-24 WEEKS)
A carefully performed second trimester ultrasound can provide more information than any other single ultrasound. This information can be categorized into 4 parts:
THIRD TRIMESTER (24-40 WEEKS)
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.
COMMENTS ON OBSTETRIC ULTRASOUNDS
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.