Ch 15 Prenatal Diagnostic Tests

Prenatal Diagnostic Test Chapter 15 Prenatal testing is used to help predict fetal outcomes and have become more accurate in recent years They can detect congenital anomalies, fetal well-being especially in risk pregnancies and allow for appropriate interventions for any problem they may find

It will also be used to provide baseline information Most women will have at least one fetal ultrasound during her pregnancy They can detect movement which includes fetal heart rate, movements and breathing efforts They have advanced over time to now have 3D and 4D versions that look very life-like Most women are excited to have an ultrasound as it can confirm in her mind that the fetus is ok by seeing the heartbeat and movements

It can put her mind at ease if she felt something was wrong but the ultrasound shows everything is ok It can cause anxious feelings in these situations so not every women is excited to have the ultrasound An ultrasound performed during the first trimester is often done transvaginally It can detect the presence of pregnancy and location, if there is more than 1 fetus, can estimate gestational age, if it is a viable pregnancy and it can detect certain anomalies Ultrasounds done in the second and third trimester are done transabdominally

During the second trimester, the woman usually needs a full bladder to to displace the intestines to give a better view of the uterus She must drink several glasses of water and not urinate until after the procedure They are done during these trimester for various reasons including confirmation of a viable pregnancy, evaluating the anatomy of the fetus, estimated gestational age, check fetal growth, where is the placenta located (it should be at the top of the uterus) and where the umbilical cord is inserted It can also tell which position the fetus is in This is necessary if the baby is not in the head down position known as cephalic position if it is close to the end of pregnancy

It is also used if an amniocentesis is necessary to guide the needle placement The advantages include it is a safe procedure and it allows visualization of the fetus and uterus The biggest disadvantage is they don’t always detect every problem I have witnessed many times when anomalies are not caught on ultrasound and not known until birth Alpha-Fetoprotein is the main protein in fetal plasma and is excreted into the amniotic fluid

Some will cross the placental barrier and enter the maternal bloodstream This means it can be tested via maternal blood sample or a sample of the amniotic fluid via amniocentesis An elevated amount of AFP can be correlated with serious fetal anomalies and requires further testing to confirm As gestation progresses levels increases so the sample should be obtained between 16 and 18 weeks Multifetal pregnancies will have higher levels as well and so can maternal obesity

This means that just because a positive result is obtained, it doesn’t always mean there is a problem Further testing is needed Down Syndrome is associated with low levels of AFP The most common cause of the elevated AFP levels is neural tube defects which can include spina bifida When doing the maternal alpha-fetoprotein screening known as MSAFP then only a small blood sample is needed

It is not very pricey and it provides parents with information regarding status of fetus and decisions can be made on if they will continue with pregnancy Limitations include it is just a tool and many factors can cause false positive which can cause worry to the parents It can’t detect all issues and timing of the test is crucial for more accurate results Some women refuse the test as they know the results may not be accurate and they have no intention of terminating the pregnancy no matter what the results will show If abnormal results are obtained, an amniocentesis would be the next step to take

The Multiple Marker screening is where they are adding different markers to the the AFP These include hcg and estriol and help detect trisomy 18 and 21 A fourth marker is inhibin A and is more accurate in detecting trisomy 21 in women younger than 35 years of age Chorionic villus sampling can be used to detect fetal chromosomal, metabolic or DNA abnormalities between 10 and 13 weeks gestationIt can be done either transcervical where a sample of the chorionic villi is aspirated or via transabdominal where where a needle is inserted to collect chorionic tissue

It could cause heavy bleeding afterwards and the women is instructed to rest at home for several hours after the procedure The advantages include the results are obtained quicker than an amniocenteiss and is the preferred method during early gestation Since results are obtained quicker, she has more time to decide if she wants to terminate the pregnancy Of course there are going to be limitations as well There is over a 2% chance of pregnancy loss after the procedure

Also limb reduction is another thing that can happen This typically is the result of when the tissue sample is being obtained, it is taking tissue from the lmb and causing it not to develop correctly The risk of uterine infection is also there although it is a not a high risk The possibility of fetal and maternal blood crossing is something to consider as well This can cause issues if mom is Rh negative and the fetus is Rh positive

Amniocentesis is when a sample of amniotic fluid is aspirated from the amniotic sac It is not performed as routinely in the first trimester due to some complications and chorionic villus sampling can be done with quicker results The purpose for doing an amniocentesis during the second trimester is to examine the fetal cells in the amniotic fluid to identify chromosomal or biochemical abnormalities of the fetus During the third trimester the amniocentesis is performed to check lung maturity or to evaluate hemolytic disease of the fetus (we will talk about that more in detail in upcoming weeks) When they are testing for lung maturity they are doing this typically to see if it is better for the fetus to be born or remain in utero

Some women have complications and if the lungs are mature, then we can provide other measures (fluids, nutrients, oxygen) to help the infant survive so they will deliver the infant early When obtaining the fluid for lung maturity they are looking for the Lecithin/Sphingomyelin (the L/S) ratio to determine maturity They lipoproteins that make up the surfactant Remember the surfactant is what keeps the lungs from collapsing The ratio they look for is 2:1 but sometimes this doesn’t always mean they are mature enough especially with women with diabetes so they may also look for a couple phospholipids

To perform the procedure the woman is in the supine position (remember that pillow or wedge under the hip) They will place her on the fetal monitor to make sure baby is not having distress before, during and after the procedure They will use ultrasound to guide the needle into a large pocket of amniotic fluid that is not close to fetal parts or near the placenta They discard the first 1-2 mL and then about 20 mL will be aspirated They will continue to monitor for 30-60 minutes and to determine no distress in the fetus and no uterine activity (contractions) are noted

She should avoid strenuous activities but normal activity can resume in 24 hours The advantages include it is relatively safe and a quick procedure and has few reported complications Disadvantages of the second trimester testing is the length it takes to obtain the results Can be up to 2 weeks There is a small risk to the fetus or umbilical cord and a small risk for infection

It also cannot guarantee 100% accurate results There are a few ways to monitor antepartum fetal surveillance that include nonstress test, contraction stress test and biophysical profile We will take a look at each of these on the upcoming slides This test may be harder for you to understand until you listen to the podcast on fetal monitoring I recommend you come back to this after you listen to that one so it can make more sense

It is done to detect an increase in the fetal heart rate with fetal movements which indicates the fetus is receiving adequate oxygenation This increase with movements is a good finding and is called an acceleration An acceleration means the heart rate increases by at least 15 beats above the baseline for at least 15 seconds The woman is placed on the external fetal monitor knowns as the EFM She can be sitting in a chair or in a bed

Sometimes the woman is given a marker she pushed every time she feels baby move but that marker is not needed When interpreting you are looking for the heart rate to increase at least 15 beats above the baseline for at least 15 seconds in two different episodes over a 20 minute period This is called reactive or reassuring and is a good thing The test can be extended for an additional 20 minutes since baby could have been sleeping during the initial time frame A nonreactive or nonreassuring result is found if there are not two of those accelerations within the 40 minute period

This could indicate fetal hypoxia or acidosis so further evaluation is needed Advantages include it is noninvasive and painless It doesn’t take too long to complete and the results are immediately available Disadvantages is a false-positive result can be obtained and unnecessary further evaluation Vibroacoustic stimulation is something that can be added to the NST

The simulator is placed on the maternal abdomen over the fetal head and the stimulation with the vibration and sound is given for approx 3 seconds The fetus will typically show more movement which can in return confirm that reassuring NST A contraction stress (CST) can also be known as the Oxytocin challenge test can be done if the NST is non reactive They are trying to get the woman to have a few contractions to basically see if the fetus is going to tolerate labor This should only be done when the woman is near full gestational age, has no complications and no history of a c-section

The woman is placed on the EFM and one of two ways can be used to stimulate contractions The first is self nipple stimulation This is where the woman will stimulate her nipples by rolling them in her fingers This will typically release the bodies own Oxytocin which is what causes contractions If this does not work, they can give her a lose dose of Pitocin which is the synthetic version of Oxytocin

Interpretation basically looks at if the fetus is tolerating labor Are there any decelerations of the fetal heart rate If not, then the test is negative which is a good thing It means there is no uteroplacental insufficiencyI am not going to require you to know more than that

Your book outlines more specific guidelines but that is more information than you need to know for nursing school Basically know that is there are signs that the baby is not tolerating the procedure, they will stop the test and discuss having a c-section The CST is not used as often anymore since there are other options but it does allow for further evaluation after that non reactive nonstress The disadvantages include it is more time consuming than other tests and the cost can be higher


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