Anatomic Defects in the Fetal Heart

Contractions of the fetal heart muscle begin about 35 days following the last menstrual period. The motion of the tiny heart can be demonstrated by sonography when the embryo is only a few weeks old. As early as the ninth week of pregnancy, counting from the first day of the last period, the fetal heart can be detected with an ultrasound device in the lowermost portion of the mother's abdomen. Detection of fetal heartbeat as early as nine weeks depends on mother being slender and the uterus in a forward position, so that distance from the fetus's heart to the microphone is minimal. Inabilitry to pick up the fetal heart by Doptone is not really meaningful this early. Typically the Doptone will detect most fetal heartbeats by 12 weeks .The fetal heart can be heard with a stethoscope at about the twentieth week of pregnancy.

It is our preference to use the fetoscope to listen to the fetal heart, and use the Doptone only very briefly for the mother or other family members to hear the heartbeat more easily than they can hear it with the fetoscope. During prenatal examinations with the mother on her back, a jellylike contact material is placed on the handheld device and the unit is pressed gently against the mother's abdominal wall. The fetal heart has a double beat like the tick of a watch and it has a soft nonmetallic pitch. The fetal heart rate normally is between 120 and 160 beats per minute throughout pregnancy.

A common misconception is that a pregnant woman can feel the baby's heart pulsate as she lies on her back. Actually she may be sensing: any of several other phenomena:

Pulsations of her own aorta, the large main artery in the trunk, as the pregnant uterus rests upon it. This rate, being synchronous with the mother's heartbeat, is easily recognizable as her pulse.

Substantial fetal hiccups, which, except for their slower rate, could be mistaken for a heartbeat.

Baby's breathing movements in the uterus. Slender women occasionally are able to see, though not to feel, these movements. They may see the skin around their belly button moving up and down rhythmically about fifty times a minute. Again, this rate is substantially slower than the fetal heart rate.

Late in pregnancy our ability to record the fetal heart by ultrasound becomes a means of testing the baby's state of health. When the baby in good health in the last few months of pregnancy, the heart rate fluctuates, depending on whether he or she is asleep or active. If the uterine blood flow and the placental circulation have begun to fail, these fluctuations no longer occur. This is called loss of variability. Such a loss also may be a response to sedative drugs taken by the mother or to anesthetic agents used for pain relief in labor.

A severely stressed baby will respond to the stress by slowing down its heartbeat from time to time. This is usually in response to the decrease in maternal blood flow in the placenta, resulting from uterine contractions that occur in pregnancy and labor. The slowed heartbeat referred to as a deceleration. It may simply be from pressure on the baby's head from the contraction, but could have a more ominous significance.

Disorders of Heart Rate and Rhythm

Even in the absence of distress, abnormalities of the fetal heart rate and rhythm can occur. A rate as high as 220 beats per minute may be picked up on a routine prenatal visit by Doptone, although it has not produced symptoms in the mother. A rate this fast cannot be counted by listening with a stethoscope, although the listener can certainly perceive that the heart is beating faster than the normal range. This increased heart rate, called tachycardia is due to an over reactive electrical system in the heart.

A very slow heart rate, down to fifty beats per minute, is the result of incomplete conduction of electrical signals and may reflect anatomic defects in the fetal heart. Other defects in conduction may cause the fetal heart to be markedly irregular.

In each of these cases, echocardiography a special form of sonography should be used to study the muscle and valves of the fetal heart to establish an exact diagnosis to form the basis for proper treatment.