You are here: Home / Areas of Care / Placenta Clinic / Placental function testing
Share:

Placental function testing

Overview of techniques used to assess placental function, including the assessment of biomarkers in maternal blood, placental morphology ultrasound and uterine artery Doppler.

QUICK LOOK

  • The 3 components of the placental function test are: maternal biochemistry, placental morphology and uterine artery Doppler

  • Maternal biochemistry comprises the blood tests for Down syndrome and spina bifida, which are re-interpreted for placental function

  • Placental morphology examination looks at placental shape (thickness, curve linear length)

  • Uterine artery Doppler examines blood flow from the mother into the intervillous space 

Contents 

Components of the placental function test

Maternal biochemistry

Placental morphology 

Uterine artery Doppler

Summary: Placental function testing 

 

 

 

 


 

Components of the placental function test 

 

Component

What is measured?

Normal findings

Maternal biochemistry

Specific proteins in maternal blood (re-interpretation of the prenatal screening blood tests for Down syndrome and spina bifida). These values are reported as Multiples of Median (MoM).

Appropriate levels of proteins in maternal blood

Placental ultrasound

Placental morphology; size, shape and texture of the placenta

Long, thin, circular shape with homogenous texture

Uterine artery Doppler

Blood flow from the mother to the placenta via her left and right uterine arteries

Adequate placental blood flow via the uterine arteries

These tests are described in further detail below.

[return to top]

 


 

Maternal biochemistry

There are several choices of prenatal screening tests available to determine the risk of having a baby with Down syndrome and a blood test to screen for spina bifida and other birth defects. Screening tests for Down syndrome combine the results of the analysis of proteins in the mother's blood with an ultrasound of the baby's neck. These screening tests are described below.

 

First trimester screen (FTS)

The first trimester screen (FTS) is performed at 11-13 weeks’ gestation and is comprised of a blood test to measure levels of two proteins in the mother's blood: 1) pregnancy-associated plasma protein-A (PAPP-A), and 2) human chorionic gonadotropin (hCG). Furthermore, to assess baby’s risk of developing Down syndrome, maternal age and the amount of fluid at the back of the developing baby's neck (called nuchal translucency) are also used.

 

Maternal serum screen (MSS)

The maternal serum screen (MSS) blood test is performed at 15-19 weeks’ gestation and measures four proteins in the mother's blood: 1) alpha-fetoprotein (AFP), 2) total human chorionic gonadotropin (total hCG), 3) estriol (uE3), and 4) inhibin (dimeric inhibin assay or DIA).

 

Integrated Prenatal Screen (IPS)

The integrated prenatal screen (IPS) combines the elements of the FTS and MSS to provide a more accurate determination of the risk of having a baby with Down syndrome. This test has a lower rate of false-positives (that is, the test falsely predicts the syndrome while the baby is born without it), and includes the risk assessment for spina bifida (measured by levels of AFP).

Please see the Prenatal Diagnosis and Medical Genetics website for more information on how these tests are used for prenatal screening.

 

Reinterpreting serum screen results

It is important to note that these biochemical markers are currently used only in prenatal screening to indicate the risk of a chromosomal abnormality (Down syndrome) and birth defect (spina bifida). If the screening test results are high-risk for Down syndrome, the most common action is to offer a diagnostic test called amniocentesis, which will look at the fetal karyotype to determine if he or she has the associated chromosomal abnormality. Since amniocentesis carries a small risk of pregnancy complications, an alternative choice is to have a detailed fetal anatomical ultrasound examination to look for specific markers that suggest Down syndrome (such as nuchal translucency). The same ultrasound examination is performed in women with elevated AFP to look for evidence of spina bifida.

In both instances, amongst women who have abnormal levels of the mentioned biomarkers, the chances are much greater that the developing baby is normal (29/30) than has either Down syndrome or spina bifida (approximately 1/30). 

Knowing that an amniocentesis test is normal and/or that the fetal anatomical ultrasound is normal is an immense relief for parents, but does it mean that they have no risk? The answer is NO. A common explanation for false-positive testing is that the placenta is not working properly and is responsible for the abnormal protein levels in maternal blood, as shown below.

 

Multiply-abnormal test results are strongly associated with stillbirth or extremely preterm birth (<32 weeks’ gestation) due to pregnancy complications from placental insufficiency. Below is a detailed explanation of the above-mentioned biomarkers and what can be expected when their levels are found to be abnormal in maternal blood.

 

Summary of Maternal Biochemistry Tests for Placental Function at 11-19 Weeks

Protein

Abnormal values

Function and what does an abnormal value mean?

Pregnancy-associated plasma protein A (PAPP-A)

<0.35 MoM*

  • activates growth factors required for fetal growth
  • released by the early developing placenta 
  • the amount in the maternal blood reflects the size of the 'footprint' of the placenta on the uterine wall 

Alpha fetoprotein (AFP)

>2.0 MoM

  • made by fetal liver
  • in the event of a skin defect, such as spina bifida on baby’s back, it leaks into maternal blood
  • if the spinal defect is not present, high AFP may signal a "leaky" damaged placenta

Total human chorionic gonadotropin (hCG)

>4.0 MoM

  • made by the outer layer of the placenta, the syncytiotrophoblast 
  • elevated levels signal abnormal placental development and function of the syncytiotrophobast

Inhibin (or Dimeric Inhibin Assay, DIA)

>3.0 MoM

  • made by the outer syncytiotrophoblast
  • dual elevation of both hCG and DIA poses a very high risk of severe pre-eclampsia 

*MoM = Multiples of Median. This expresses how far a test value deviates from a normal population's median value at any gestational age. To understand the concept of MoM, imagine sitting next to a 9ft tall man, who is 50% taller than a 6ft tall man: the taller man's height is 1.5 MoM. Another 6ft tall man's height is 1.0 MoM.

 

A placental health examination is indicated if one or more of the following are found with a fetus that has a normal anatomy ultrasound (no open skin defects) and/or a normal karyotype (no chromosomal abnormalities):

-          PAPP-A <0.35 MoM 

-          AFP > 2.0 MoM (Mount Sinai Hospital neural tube defect cut-off) 

-          Inhibin >3.0 MoM 

-          Total hCG >4.0 MoM

This placental health examination is comprised of a placental morphology ultrasound and uterine artery Doppler.

[return to top]


 

Placental morphology ultrasound

Ultrasound is a non-invasive diagnostic tool initially developed in obstetrics to visualize and measure the developing fetus and its surrounding structures (amniotic fluid, umbilical cord, placenta, uterus, cervix, and ovaries). Ultrasound sends sound waves into the body through a transducer. These sound waves bounce off internal structures and are sent back to be interpreted and represented as an image on the monitor. The imagine is real-time and can show the developing baby as a live moving person. At present, the ideal window for the placental morphology ultrasound is 19-22 weeks (combined with, or near to, the established fetal anatomical ultrasound).

 

Placental size, shape and texture

The placental ultrasound assesses the placental morphology, which includes placental size, shape and texture. Each of these is compared between normal and abnormal placentas in the images below. 

 

 

 

What Other Abnormalities Can Be Determined Through Ultrasound Scanning?    

Placental location

In a typical pregnancy, the placenta attaches to the upper wall of the uterus (called the fundus). However, when the placenta is in the lower uterine segment of the uterus, it may cover part of all of the internal os (opening) of the cervix. A low-lying placenta near or covering the internal os is termed placenta previa. This is an important complication of pregnancy to recognize, and as such, the determination of placental location is currently part of the fetal anatomical ultrasound examination.

 

Umbilical cord insertion

At the end of the first trimester of pregnancy (12 weeks or 3 months), the placenta has become distinct from the membranes. Using ultrasound, we can visualize the development of the placenta and the location of the umbilical cord. Typically, the umbilical cord is seen to insert in the central part of the placental disc (central cord insertion). If part of the placenta becomes damaged or does not develop properly, then that area of the placenta thins out into membranes leaving the umbilical cord near the edge of the placental disc (marginal cord insertion: the cord inserts into the edge of the placenta or velamentous cord insertion: the cord inserts into the membranes, not into the placenta). Velamentous insertion increases the risk of developing vasa previa during pregnancy.

 

Number of blood vessels in the umbilical cord

The umbilical should have three blood vessels; two umbilical arteries and one umbilical vein. The umbilical arteries transport deoxygenated blood and waste from the fetus to the placenta. The umbilical vein carries oxygenated blood and nutrients from the placenta to the fetus to support growth and development. Occasionally, the umbilical cord contains only one umbilical artery (called a two-vessel cord). When this occurs, the remaining umbilical artery is wider, and thus has a compensatory larger volume of blood flow. Sometimes, this compensation does not occur, and the developing baby's growth may be restricted. Conversely, a single umbilical artery may have nothing to do with abnormal placental development and may be associated with birth defects (for example, a missing kidney) or other more serious problems. Therefore, pregnancies with a 2-vessel-cord require careful ultrasound examination.

[return to top]


 

Uterine Artery Doppler

Uterine artery Doppler measures the blood flow to the placenta via the maternal uterine arteries. It uses high frequency sound waves and their echoes to visualize blood flow. This ultrasound uses a probe to transmit high-frequency sound pulses into your body. Once a pulse is sent, the probe waits for the pulse to hit a boundary and be echoed back. The frequency that the pulses return to the probe is interpreted and presented as an image or as a graph. The analogy is to remember the change in sound from a train that approaches and continues past you in a train station without slowing down. The similar information obtained from the Doppler ultrasound can be displayed in two complementary modes:

  • An image (colour Doppler) [the upper portion of each image below] – superimposed on an ordinary ultrasound, this Doppler shows blood moving away from the probe as BLUE and blood moving towards the probe as RED
  • Graphically (spectral Doppler) [the lower portion of each image below] – represents the changing blood flow through the cardiac cycle and allows for semi-quantitative measurements. From this graph, the echoes are interpreted as one high peak followed by a lower peak; together this represents a heartbeat. The higher peak corresponds to peak systolic velocity (blood flow when the heart has contracted). The lower peak corresponds to end diastolic velocity (blood flow when the heart is relaxed).

 

Normal uterine artery Doppler

During a healthy pregnancy, maternal blood vessels feeding the placenta develop and relax to lower resistance, thus, increasing the blood flow into the intervillous space of the placenta. As pregnancy progresses, the early diastolic notch (or dip in the waveform) disappears and the PI decreases as more blood flows through the developed uterine blood vessels during the diastole. This is illustrated in the top and bottom panels in the image below.

 

Abnormal uterine artery Doppler: persistent diastolic notch

In some pregnancies, the early diastole notch may persist, potentially due to the vessels not enlarging or dilating as they should. The persistent diastolic notch is illustrated in the bottom panel of the image below. Such a finding means that there is lower blood flow and less nutrient-rich blood for the developing baby. We call this utero-placental vascular insufficiency (UPVI) and it implies a restricted blood supply (see abstract by Drs. Kingdom and Burton, published in Placenta, June 2009). This also poses a significant risk factor for long-term maternal cardiovascular disease. Therefore, women who develop pre-eclampsia and/or IUGR or deliver at <32 weeks always require follow-up of their own health, even when their blood pressure returns to normal (see abstract by Yinon et al, published in Circulation, November 2010, and additional evidence).

 
Pulsatility index

The standard method of measurement of Doppler waveforms is the pulsatility index (PI). The pulsatility index reflects the impedance (the pulsatile resistance in an elastic vessel) in the uterine arteries. During pregnancy, the uterine arteries enlarge and dilate; this is termed as decreased resistance, as it permits a larger volume of blood to be diverted to flow into the intervillous space of the placenta.

Therefore, a LOW pulsatility index is equivalent to LOW resistance. This means that a large volume of blood can easily travel to the placenta through the uterine arteries. On the other hand, a HIGH pulsatility index is equivalent to HIGH resistance, meaning that it is more difficult for blood to travel to the placenta through the uterine arteries compromising the amount of oxygen and nutrients delivered to the baby.

 

Changes in PI values across gestation

At the 19-22 week window, the mean PI of the right and left uterine arteries should be below 1.45. A mean PI greater than 1.45 indicates impaired uterine artery blood flow. The change in mean PI across gestation is illustrated in the figure below. Note the progressively decreasing mean PI values obtained throughout gestation which demonstrate the expected changes over the course of pregnancy.

                   

The shape of the Doppler waveform is also important. If there is a dip following the higher peak, we call this an early diastolic notch (see an example in an image above). These notches are normally present in early pregnancy and should have disappeared by 19-22 weeks’ gestation as blood flow increases. If these notches persist at the 19-22 week placental ultrasound, this suggests an impaired maternal blood supply to the developing placenta.

Our research demonstrates that women with bilateral abnormal uterine artery Doppler are 7 times more likely to have a small placenta (70% vs. 10%); the combination of a small placenta and abnormal uterine artery Doppler confers a high risk of preterm delivery (<32 weeks) due to combinations of pre-eclampsia and fetal growth restriction (see abstract by Toal et al, published in the American Journal of Obstetrics and Gynecology, March 2008).

 

The benefit of second trimester uterine artery Doppler

Medical research literature on screening for placental insufficiency is dominated by a focus on identifying women at risk of early pre-eclampsia by combining the results of early uterine artery Doppler (at 11-13 weeks with the nuchal translucency examination) with additional blood tests.

Our view is that there is really no urgency to identify this subset of women so early, since at present, no effective intervention exists. Women judged to be at risk of pre-eclampsia should be started on low-dose aspirin (81mg/day) as prophylaxis before 16 weeks of gestation (see reference). Our philosophy is that a delay in screening allows greater use of pre-existing information (ex. the maternal biochemistry) and when combined with both placental morphology and uterine artery Doppler, this information can be used to assign the risk for perinatal problems attributable to placental insufficiency rather than focusing on just one aspect (see abstract by Toal et al, published in the American Journal of Obstetrics and Gynecology, April 2007).

[return to top]


 

Summary of placental function testing 

 

 

Evidence for Placental Function Testing

The extensive research linking the prenatal blood test results (described above in maternal biochemistry) to adverse pregnancy outcomes has been summarized by a research method called systematic review and meta-analysis. Under this analysis, only two blood tests (elevated AFP for IUGR and stillbirth and elevated inhibin for severe pre-eclampsia) achieved sufficient test characteristics to be justified as blood tests for placental function. We agree with these findings and, hence, re-interpret the blood test results in their original MoM values in order to screen for placental insufficiencies, as opposed to merely looking at the risks of neural tube defect or Down syndrome.

Research into the usefulness of the other biomarker (PAPP-A) is more limited, and the screening test for PAPP-A levels was introduced in North America only in 2004, as a wide-scale screen for the detection of Down syndrome. However, PAPP-A has received more attention recently, particularly within our research group. We studied the outcomes of 90 pregnancies characterized by very low PAPP-A levels where the baby’s karyotype was normal (ie, no Down syndrome). To support our hypothesis that abnormally low PAPP-A levels are indicative of placental pathology, we found that approximately 1 in 4 women was found to have a small, thick and abnormally-shaped placenta during the ultrasound screen. This finding was associated with the subsequent risks (see placental complications for list of these conditions). Interestingly, our data suggested that the placental shape is much more important than the uterine artery Doppler test to identify women with low PAPP-A at the most risk (see abstract by Proctor et al, published in Ultrasound in Obstetrics and Gynecology, September 2009).

[return to top]

 

Document Actions