Ultrasound is the only widely used imaging modality that appears to be safe during pregnancy, and it has been estimated that it is currently used in 20% of pregnancies in the U.S.
Ultrasound can diagnose pregnancy at least as early as urine tests and certainly is more accurate. At four to five weeks after the last menstrual period (LMP) a tiny so-called gestational sac appears in the uterus. This hollow fluid-filled ball which looks like a lima bean which is approximately eight mm long, represents the embryonic membrane. The embryo inside is still so minute that it is below the resolving power of the sound waves. (Actually the embryo is really only two to three weeks old at this stage because fertilization occurs at about two weeks after the onset of the last menstrual period.
By six weeks LMP the embryo has grown enough to begin to give off echoes. By the use of real-time ultrasound it can be seen moving in the gestational sac. At seven weeks a beating heart can be detected.
An important use of ultrasound is to detect ectopic (tubal) pregnancies. These pregnancies, which of course can never come to term, are dangerous to the mother because they will rupture the fallopian tube if not detected. If a woman has a positive urine test but ultrasound does not demonstrate a gestational sac in the uterus, then the diagnosis of tubal pregnancy is fairly assured.
Ultrasound is most commonly used during pregnancy to determine the due date of the baby. This might not seem like a particularly crucial reason, but according to Van Bergen, “ . . . there is a fourfold increase in perinatal mortality in pregnancies with uncertain dates.”
Pregnant teenagers whose periods are very irregular may be especially unsure of their expected delivery dates, and therefore constitute a high-risk population. The obstetrician needs to know the age of the fetus for several reasons. For instance, if a caesarean has to be done the doctor has to be sure that he/she does not deliver the baby before its lungs are mature enough to breathe on its own. Or if labor should start spontaneously the doctor must know whether it is premature and therefore should be stopped if possible.
The method used to determine the age of the fetus is as follows: by use of ultrasound the fetal skull is located and then with electronic calipers the transverse diameter is measured on the screen. This measurement is called the biparietal diameter (BPD). The doctor then uses a chart of BPD measurements to correlate the fetal head diameter with its age. For instance BPD measurement of the fetus in the sonogram below was 2.4 cm. On the basis of this (and also of the measurement of the femur) the baby’s age was put at 13 weeks.
13 week old fetus
(figure available in print form)
Sonogram courtesy of the Yale-New Haven Hospital Ultrasound Lab
Biparietal diameter charts , which are based on thousands of measurements, have such a high degree of reliability because of the fact that babies’ heads all seem to grow at the same rate in
, regardless of race, ethnic origin or size of parents. Even intrauterine starvation of a fetus usually does not slow down the growth of the head, although the rest of the body may be gaunt. One reservation must be noted however. The use of BPD measurements to determine age is much more accurate early in pregnancy than later; after 30 weeks the range of variations is so great that the measurement is not useful.
When a patient comes in to the lab for a BPD measurement to determine her due date, the ultrasound technician routinely checks the organs of the fetus to make sure that everything is normal. It is amazing to realize that ultrasound can even image the soft organs of a fetus inside its mothers uterus:
According to the ultrasound technicians at Yale they check the following fetal organs: kidneys and bladder for obstructions; the heart (which can be seen beating); the spine for a condition known as spina bifida; the head for possible hydrocephaly (enlargement of the brain due to excess pressure); the stomach and the limbs.
Occasionally these routine exams can lead to dramatic results. A few fetuses who were diagnosed as being hydrocephalic during ultrasound viewing have actually had shunts placed in their heads, in
, in order to drain off the fluid which caused pressure on the brain. In another case, a male fetus was seen to have an enlarged bladder caused by a urethral obstruction. Again a shunt was placed
in order to relieve pressure and future organ destruction.
Those are the dramatic cases that involve invasive procedures, but babies’ lives are sometimes saved in less sensational and dangerous ways. Three fetuses at Yale-New Haven Hospital were found during ultrasound exams to have abnormal and potential lethal heart rhythm disturbances. The mothers were subsequently given digitalis, a powerful heart regulator, and the dysrhythmias were corrected without any surgery.
Longitudinal view through heart chambers of fetus
(figure available in print form)
Reprinted by permission of Dr. K.J.W. Taylor, from
“Atlas of GrayScale Ultrasonography
”, © 1978 by Churchill Livingstone
Ultrasound is also used to determine multiple births. The diagnosis of twins, for instance, is regarded as positive when the sonographer can locate two skulls in the same plane. The task becomes much more difficult when there are more than two fetuses.
The position of the fetus can also be imaged on ultrasound. Before 28 weeks the fetus can move around fairly freely in the uterus but after that it assumes its fetal lie. The normal position is cephalic presentation (head down) but occasionally the baby is in a breech position (buttocks down) or transverse position (lying at a 90’ angle to the cervix). It is necessary for the obstetrician to be aware of these presentations so that a caesarean can be performed if indicated.
In addition to the position of the fetus, the location of the placenta can also be visualized. This is important, especially in the case of placenta previa, a condition where the placenta covers the opening to the cervix . In this situation, the woman would hemorrhage during a vaginal delivery, so if ultrasound does show placenta previa at term then a caesarean would be indicated.
The doctor also needs to know the location of the placenta and fetus when amniocentesis is required. This procedure is routinely offered to pregnant women who are over 35 and who are therefore at greater risk for bearing a child with Down’s Syndrome. A baby with this condition has 47 chromosomes instead of the normal 46. The procedure which is done when the fetus is approximately 16 weeks old, involves the insertion of a hollow needle through the woman’s abdomen into the amniotic sac to obtain a sample of fluid with fetal cells. These cells are then cultured and examined for the extra chromosome. The accuracy and safety of amniocentesis procedures depends on the use of ultrasound.
A particularly fascinating aspect of ultrasound is that it allows us to watch the movements of the fetus inside its mother’s uterus. It can be seen sucking its thumb, hiccuping and even making breathing movements after 20 weeks. This “breathing” is probably just practice for the outside world since the fetus is well supplied with oxygen through the umbilical cord.
According to Dr. Hobbins the fetus at term normally spends about 70% of its time in a REM (rapid eye movement) state of sleep. (When we adults dream during sleep, we normally exhibit REM.) Ultrasound actually allows the viewer to watch the movement of the lenses inside the fetus’ closed eyes. If however, the fetus is stressed there is no REM sleep and its absence at term is an indication of trouble.