Obstetrics

Prenatal Cytogenetic Testing: MSAFP, Quad Screens, and Amniocentesis

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Prenatal Cytogenetic Testing

The goal of all prenatal testing is to determine if there are any abnormalities in the developing fetus. Not all pregnancies are "candidates" for certain forms of prenatal testing. Determining whether or not to perform invasive testing depends on many factors. Some of the common indications for invasive prenatal cytogenetic testing include:

(1) Advanced maternal age; mother will be greater than or equal to 35 years of age at time of delivery.
(2) Previous child with a genetic abnormality, especially if mother is greater than 30 years old.
(3) Known parental chromosomal abnormalities.

MSAFP

The first test that is often ordered is the maternal serum α-fetoprotein (MSAFP). The MSAFP is typically ordered between 15 and 20 weeks gestation. AFP is produced by the fetus. It crosses the placenta into the maternal blood stream where it can be measured. MSAFP can be normal, elevated, or decreased.

Elevated levels of MSAFP occur for many different reasons. Some are pathological, others are not. It is associated with neural tube defects (ie: spina bifida, anencephaly, etc.), gastrointestinal and abdominal wall problems (ie: gastroschisis, omphalocele), multiple gestations (ie: twins, triplets, etc.), fetal death, placental problems, as well as underestimated gestational age.

Depressed levels of MSAFP are harbingers of chromosomal abnormalities. The most common one is trisomy 21 (Down’s syndrome). In addition, trisomy 18 (Edward’s syndrome) is also associated with low levels of MSAFP.

Quad Screen

If abnormalities in MSAFP are detected additional maternal blood markers are measured. This is referred to as the "quad screen".

The quad screen consists of MSAFP, estriol, inhibin-A, and beta-HCG. The levels of these molecules taken together provide greater sensitivity in diagnosing chromosomal abnormalities in the fetus. For example:

Fetal Abnormality: Quad Screen Results:
Trisomy 21 (aka: Down’s Syndrome)

– Decreased AFP and estriol

– Increased β-HCG and inhibin
Trisomy 18 (aka: Edward’s Syndrome) – Decreased AFP, estriol, inhibin, and β-HCG

An abnormality in any of these markers is not 100% diagnostic of fetal pathology. In order to confirm the fetal diagnosis more invasive tests can be performed. These include amniocentesis and chorionic villus sampling.

Amniocentesis

Amniocentesis collects amniotic fluid surrounding the developing fetus. Amniotic fluid contains fetal cells that can be sent for genetic analysis. It is typically done around 15 to 17 weeks of gestation.

The procedure involves using a needle to withdraw the fluid. This is done either through the abdomen or cervix. Since amniocentesis is an invasive test there are risks. Miscarriage can occur in up to 0.5% of cases; in addition, bleeding (either maternal or fetal) can also occur.

Indications for amniocentesis Advanced maternal age (>35 years old) at time of delivery.
If quad screen was abnormal.
In cases where maternal-fetal blood incompatibility is an issue (ie: Rh-sensitization).
In cases where premature delivery may occur. Amniocentesis can be done to help determine the maturity of the fetal lungs.

Chorionic Villus Sampling

Chorionic villus sampling is another invasive test that can be used to diagnose fetal chormosomal abnormalities. It is typically done between 10 and 12 weeks of gestation. It involves sampling a piece of the chorion, which is a component of the placenta. Its main advantage is that it can be done earlier than amniocentesis; however, there is a slightly increased risk of fetal death. In addition, the diagnostic accuracy is not quite as great as amniocentesis in picking up neural tube defects.

Peripheral Umbilical Blood Sampling

Peripheral umbilical blood sampling is a process in which the fetal umbilical blood vessels are punctured so that fetal red blood cells can be obtained for analysis. This procedure is typically done in the 2nd or 3rd trimesters when there is a possibility of fetal hemolytic disease. Hemolytic diseases of the newborn occur when the mother produces antibodies that attack fetal red blood cells.

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Basics of Pregnancy: GxPy, Naegele’s Rule, and that Bundle of Joy

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This article will discuss the basics of pregnancy. In order to understand the basics we have to understand two important obstetric terms: gravity and parity.

Gravity refers to the number of times a woman has been pregnant. Parity refers to how many times the woman has given birth to an infant older than 20 weeks gestational age, or weighing greater than 500 grams.

On a medical chart a woman’s obstetric history is commonly written as “G x P y”. “G” refers to the gravity, or the number of times, “x”, a woman has been pregnant. The “P” stands for the parity, or the number of times, “y”, a woman has given birth to an infant older than 20 weeks gestation, or weighing greater than 500 grams. In other words the “y” equals the number of pregnancies that made it past 20 weeks gestation.

However, a woman’s obstetric history may also contain miscarriages, abortions, preterm births, and still births. In addition, some of her children may have died early in infancy. To account for these facts the “Gx Py” system is sometimes written as “G x P y, a, b, c, d”. The “a” refers to the number of term pregnancies; the "b" refers to the number of preterm pregnancies. The "c" refers to the number of abortions; abortions includes both spontaneous and induced, as well as ectopic pregnancies. And finally, the "d" refers to the number of living children the woman currently has.

So, as an example, a woman with two living children and one previous spontaneous abortion at twelve weeks gestation, who is currently pregnant for the third time would have an obstetric chart that reads "G 3 P 2, 2, 0, 1, 2."

Fetal Age nd Due Date

Pregnancy Wheel
There are different measures of fetal age. They include the following:

(1) Developmental age
(2) Gestational age

The developmental age is the age of the fetus measured from the time of fertilization. Since it is often difficult to determine exactly when fertilization occurred, a surrogate marker is used.

The surrogate marker is the gestational age. The gestational age is measured from the first day of the last menstrual period. Typically, the gestational age overestimates the developmental age by approximately two weeks.

If the last menstrual period is not known there are other ways of determining the gestational age. One method is to use fundal height; fundal height is the location of the fundus (ie: the top most portion of the uterus) as felt by palpating the maternal abdomen. By 20 weeks of gestation the fundal height should be at the mother’s umbilicus (belly button). From there on, the height roughly correlates with gestational age (ie: at 28 weeks the fundal height is 28cm above the pubic symphysis).

The second method, albeit an even more imprecise one, is to use quickening. Quickening is the detection of fetal movements by the mother. It usually corresponds to a gestational age of 17 to 18 weeks. Ultrasound can also be used to determine the gestational age, and is the most reliable way, assuming the last menstrual period is not known.

In order to calculate the delivery date a simple mathematics trick known as Nagele’s rule is used. It states that the estimated delivery date is determined by adding nine months plus seven days to the first day of the last menstrual period.

Naegele’s rule -> Last menstrual period + 9 months + 7 days = expected due date

Pregnancy Tests

How exactly do you determine if someone is pregnant? The most common way is to use a urine pregnancy test. These tests detect a molecule known as beta-HCG (ie: beta human chorionic gonadotropin). Beta-HCG is a molecule secreted by the placenta. Its level doubles roughly ever 48 hours during the first few days of pregnancy. It peaks at about 10 weeks of gestation. Urine tests tend to have a high false negative rate. This means that they miss pregnancies, especially if the pregnancy has just begun.

A more sensitive test is the serum (ie: blood) pregnancy test. This test measures exactly the same molecule, beta-HCG, but it does so from a blood, rather than a urine sample. This test can pick up pregnancies earlier than a urine test can.

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Bun in the Oven: Fetus Surveillance Techniques

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Fetal growth is most commonly monitored by ultrasound. Most “routine” pregnancies begin with one fetal ultrasound between 18 and 20 weeks of gestation. This ultrasound can pick up important defects that may not be apparent at later gestational dates. If the 18-20 week ultrasound is normal, and the pregnancy is otherwise uncomplicated, repeat ultrasounds are not routinely performed.

Fetus

Fetal growth can also be monitored by measuring the "fundal height". The fundal height is the distance between the fundus (ie: top of the uterus), which is felt via palpation of the abdomen, and the pubic symphysis (see image to left).

A rule of the thumb is that the top of the uterus should be felt at the level of the umbilicus (ie: belly button) at approximately 20 weeks gestation. It then increases proportionally until about 36 weeks gestation. At this point the height may actually decrease as the fetal head enters the pelvis.

Fetal Maturity

There are three different ways to assess fetal maturity. They are used to determine if the fetus has adequate lung maturity to survive outside the uterus. These tests measure molecules that are present in the amniotic fluid. Therefore an invasive test known as an amniocentesis is necessary. The different methods are:

(1) Lecithin to sphingomyelin ratio
(2) Phosphatidylglycerol test
(3) Foam stability index

The first way of determining fetal lung maturity is to measure the lecithin to sphingomyelin ratio. Sphingomyelin is a specialized phospholipid that is produced in constant amounts by the fetus throughout pregnancy. Lecithin is a component of surfactant (ie: molecules that help keep the alveoli of the lung inflated) and its production increases as fetal lung maturity increases. An L:S ratio of greater than two indicates that the fetal lungs are mature enough to work outside the uterus.

The second test is known as the phosphatidylglycerol (PG) test. The presence of this molecule in the amniotic fluid indicates fetal lung maturity. PG is a component of pulmonary surfactant, which helps keep the gas exchange portions of the lung (ie: alveoli) from deflating at the end of exhalation.

The third test is the "foam stability index" test. This test measures the stability of bubbles when amniotic fluid is shaken with variable percent alcohol solutions. If the bubbles are stable this indicates fetal lung maturity.

Fetal Well-Being

Fetal well-being is assessed using several different methods. They include:

(1) Non-stress test
(2) Stress test
(3) Biophysical profile

Then on-stress test measures fetal well-being by looking at changes in the fetal heart rate during movement. A normal (aka: "reactive") non-stress test occurs when the fetus’ heart rate increases by 15 beats per minute over 15 seconds following fetal movement. If this increase in heart rate occurs twice within 20 minutes of monitoring the test is normal or “reactive”. Note: if the fetus is less than 32 weeks gestation then only a 10 beat per minute increase is needed to satisfy the non-stress test.

The stress test (aka: "contraction stress test", or "oxytocin stress test" depending on how contractions are elicited) measures the fetus’ ability to handle the "stress" of a uterine contraction. When the uterus contracts blood flow from the placenta is momentarily decreased. A healthy fetus will have no discernible change in their heart rate; however, an at risk fetus will have heart rate decelerations. Stress tests, especially oxytocin induced stress tests, commonly result in false-positives (ie: indicate fetal jeopardy when the fetus is normal); therefore, an abnormal stress test is followed by a non-stress test. If the non-stress test is normal/reactive then the stress test is often times considered a false-positive.

The biophysical profile uses five parameters for assessing fetal well-being. They include: fetal breathing movements, gross body movements, fetal muscle tone, fetal heart rate as measured by non-stress test, and qualitative amniotic fluid levels. Each of these parameters is given a score of 0 if absent/abnormal, or 2 if normal. A score of 8-10 is reassuring and indicates no problems with the fetus.

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