Antepartal Fetal Surveillance

The following is taken from an update given to Ob/Gyn board candidates at the LSU Board Review in April, 1998. It is intended to be a basic review of the topic.

"The rate of the foetal heart is subject to considerable variations which affords us a fairly reliable means of judging as to the well-being of the child." This was excerpted from Dr. J. Whitridge Williams, in the second edition of Williams Obstetrics, in 1908. Much has been learned in the 90 years since that statement was written, we still rely on the fetus to give us insight into its health, but we have many more sophisticated methods to evaluate the fetus, and I will try to highlight them in this talk.

The purpose of antepartal fetal surveillance is to determine the well-being of a fetus at risk for antepartal fetal loss. We can do this by reducing the risk of fetal death in utero, delaying the need for inappropriate interventions, and prolonging the gestation in pregnancies at risk for preterm delivery.

There are a litany of medical and obstetric diseases that can risk fetal death, and thus lead us to intensify fetal observation. These include the hypertensive disorders, pre-eclampsia, eclampsia and chronic hypertension. Other medical diseases also enter the picture, such as insulin dependent Diabetes Mellitus, chronic renal diseases, hyperthyroidism, lupus, cyanotic heart disease, and hemoglobinopathies. Fetal disorders add to the list. These are multiple gestations, post-term pregnancies, growth restriction disorders, amniotic fluid disorders, decreased fetal movement, isoimmunization and hydrops, and previous unexplained fetal death in utero.

The advent of antepartal testing excited us as a means to prevent untimely fetal death. But, there is NO one best test. With a normal or reassuring test, we have learned that it is unlikely that there will be a fetal death within the next five days. Additionally, an abnormal or reassuring test has a high rate of false positives, and requires retesting or going onto a different testing method. There is also no defined time to start testing, nor is the interval of testing firm. Most authorities begin at 32 weeks, and the interval of testing is generally weekly to twice weekly.

Methods of Antepartal Fetal Surveillance

Discussion of fetal testing must begin with fetal kick counts. Sadovsky and Yaffee discovered in 1973 that their pre-eclamptic patients noticed decreased fetal movement prior to encountering a fetal demise. This led them to start fetal kick counts daily in their patients that were high risk due to problems with placental insufficiency. Other researchers have found flaws in this form of testing. Ultrasound studies show that a mother can feel up to 80 percent of movements seen on scan, but others point out that after 36 weeks, a mother may feel only 15 percent of movements. There is also no defined number of movements that must be felt, nor is it known over what time frame the testing should occur.

The Non-Stress Test (NST) began flexing its muscles in the 1960's. Hammacher et al began the study of the NST as a test of the fetal condition. The physiology behind the NST involves the fetal sympathetic and parasympathetic nervous systems and fetal baroreceptors. The sympathetic system is in the upper cardiothoracic nerves. These are acceleratory centers in early pregnancy, and deceleratory centers as the fetus matures. The parasympathetic system works to slow the heart rate throughout pregnancy. The aortic and carotid baroreceptors are responsible for moment to moment beating, and the chemoreceptors are stimulated by oxygen tension such that with acute hypoxia there will be increased variability, and with chronic hypoxia there will be decreased variability. Thus the value of the NST is that in a non-acidotic, neurologically intact fetus, the heart rate should accelerate with fetal movements.

The Non-Stress test is performed with the patient in semi-fowlers, and is best performed soon after a meal. A REACTIVE test is one that shows two 15 beat accelerations of the fetal heart rate accompanying movement in a 20 minute period of time. Each acceleration must last 15 seconds. A NONREACTIVE test is one that shows no accelerations in a 40 minute period of time.

The Contraction Stress Test (CST) or Oxytocin Challenge Test (OCT) was first introduced by Ray in 1972. It is said to be a test of the uteroplacental unit. In 1975, Freeman introduced the parameters of contraction number and frequency to standardize the test. Huddleston introduced the concept of Nipple Stimulation as an adequate means of performing the test in a less cumbersome fashion. The value of the CST is that if fetal oxygenation is marginal at rest, it will transiently worsen with uterine contractions ans the resultant hypoxemia will lead to late decelerations. It was also said that if variable decelerations were seen, one should suspect oligohydramnios.

The CST is performed with the patient in semi-fowlers position. If the patient is not having spontaneous contractions, pitocin is begun at 0.5-1.0mU and increased every 15-20 minutes until an adequate contraction pattern is obtained. This pattern is 3 contractions in a 10 minute period of time. Nipple stimulation involves intermittent nipple massage to accomplish the same goals. A problem with the testing is the length of time it may take to perform the test, up to 90 minutes in some cases.

A NEGATIVE test is no decelerations with the 3 contractions in the 10 minute window. A POSITIVE test is late decelerations with 50% or more of the contractions. A SUSPICIOUS test has intermittent late decelerations or severe variables. An UNSATISFACTORY test has less than 3 contractions or hyperstimulation.

The Biophysical Profile (BPP) was first described by Manning in 1980. It utilizes multiple ultrasound observations and the NST. It is more accurate than a single test as it correlates 5 measurements to give a score, thus fewer false positives and false negatives. The parameters of the test are fetal tone, fetal movement, fetal breathing, and amniotic fluid volume. An NST rounds out the variables, and is not an ultrasonic measurement.

Two points are given if the observation is present and zero points are given if it is absent. The following list shows the parameters of each observation.

A BPP test score of at least 8 out of 10 is considered reasurring. A score of 6 or 7 out of 10 is equivocal, and must be repeated within 24 hours. A score of 4 or less out of 10 is a positive test and strongly suggests preparing the patient for delivery.

Since its inception, various authors have modified the BPP. In 1987, Vintezelos stated that the first sign of acidosis (then defined as cord arterial pH < 7.2) is an abnormal NST and absent fetal breathing. Advanced or chronic acidosis shows compromised fetal tone and movement. We also know that Amniotic fluid volume is a quick evaluation of long term uteroplacental function as in the late 2^nd and all the 3^rd trimester, amniotic fluid is essentially fetal urine. Thus with placental dysfunction, there is decreased renal perfusion and thus oligohydramnios. Manning described the modified BPP in 1990, combining an NST, amniotic fluid volume and fetal breathing. It is less cumbersome than the original BPP and its results are just as predictive. A more recent suggested biophysical evaluation utilizes only amniotic fluid volume by the four quadrant index (as it is felt to be more accurate of the total volume) and the NST. The Fluid index is the indicator of long term uteroplacental function and the NST is the short term indicator of fetal acid-base status.

In summary, the practicing grass roots OB/GYN has many weapons in his arsenal to assess fetal health and well being. These are well rooted in fetal physiologic responses. It should be remembered that there is no one best test and that none of the testing modalities has been shown to reduce perinatal morbidity nor to increase long term fetal neurologic outcome.