Placental Function Testing
Placental Function Testing
Components of the Placental Function Test:
|Component||What is measured?||Findings in a normal, healthy placenta|
|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 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|
Prenatal screening for Down syndrome and spina bifida
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 describd below:
First trimester screen (FTS)
This 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) free human chorionic gonadotropin [free hCG]. The mother's age and an ultrasound to measure the amount of fluid at the back of the developing baby's neck (called nuchal translucency) are also used.
Maternal serum screen (MSS)
This 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)
This test combines the elements of the FTS and MSS tests to provide a more accurate (less risk of a false positive test) determination of the risk of having a baby with Down syndrome. It includes the same risk assessment for spina bifida and other birth defects, since it includes AFP.
Please see the Prenatal Diagnosis and Medical Genetics website for more information on using these tests for prenatal screening.
The results from these tests can be re-interpreted for placental function
It is important to note that these biochemical markers are only currently used in prenatal screening to indicate the risk of a chromosomal abnormality (Down syndrome) and birth defect (spina bifida). If the screening tests results are high-risk for Down syndrome, the most common action is to offer a diagnostic test called amniocentesis. This will determine if the developing baby has Down syndrome or not. 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. The same ultrasound examination is performed in women with elevated AFP, to look for evidence of spina bifida.
In both instances, amongst women who test positive, 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 normally, 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.
Summary of Maternal Biochemistry Tests for Placental Function at 11-19 Weeks
|Protein||Name||Function||Abnormal values||What does an abnormal value mean?|
|PAPP-A||Pregnancy-associated plasma protein A||Activates growth factors required for fetal growth
||0.35 MoM* or less
||PAPP-A is released by the early developing placenta. Thus, the amount of PAPP-A in the maternal blood reflects the size of the 'footprint' of the placenta on the uterine wall
|AFP||Alpha fetoprotein||This protein is made in the developing baby's liver
2.0 MoM or greater
|AFP is a large protein that is made by the baby's liver. It leaks into maternal blood across any skin defect, like a spina bifida lesion on the baby's back. With a structurally normal fetus, elevated maternal AFP levels may be due to a "leaky" damaged placenta|
|Total hCG||Total human chorionic gonadotropin||This protein is made by the outer layer of the placental villi||4.0 MoM or greater||hCG is made by the outer 'skin' of the placenta, called the syncytiotrophoblast. This layer also makes the anti-angiogenic protein sFlt1 that is largely responsible for causing the hypertensive disease pre-eclampsia
|DIA||Inhibin (or Dimeric Inhibin Assay)||This protein is made by the outer layer of the placental villi||3.0 MoM or greater||Inhibin is also 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.
- 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
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 ultrasound determines the placental morphology – the placenta’s size, shape and texture.
Size and Shape
The placenta should appear long (>12cm in length) and thin (<3cm in width) in size
Long and thin
Thick and small
The placenta should be a smooth, consistent (homogenous) texture
Smooth consistent texture
Abnormal, heterogenous texture
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.
Abnormal 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. Typically, the umbilical cord is seen to insert in the central part of the placental disc. 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. Using ultrasound, we can visualize the development of the placenta and the location of the umbilical cord.
Marginal cord insertion: the cord inserts into the edge of the placenta
Velamentous cord insertion: the cord inserts into the membranes, not into the placenta. If this occurs, the woman is at risk of developing vasa previa.
Abnormal 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.
Uterine artery Doppler 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).
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 decreased resistance, as it permits a larger volume of blood to be diverted to flow into the intervillous space of the placenta.
- 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.
- A HIGH pulsatility index is equivalent to HIGH resistance. This means that it is more difficult for blood to travel to the placenta through the uterine arteries, and thus the placenta (and fetus) are receiving less blood than they should be.
Changes in PI values during normal pregnancy
|Note the two values obtained at 18, 22, and 24 weeks of gestation, demonstrating the expected changes over the course of the pregnancy.|
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 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 images below). 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 placenta 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 IUGR. (See abstract, article by Dr Toal, published in the American Journal of Obstetrics and Gynecology, March 2008)
Uterine artery Doppler
Maternal blood flow to the placenta via the uterine arteries
Normal uterine artery Doppler in a pregnant woman
||During the pregnancy, maternal blood vessels feeding the placenta develop and relax to lower resistance thus increasing her blood flow to 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 utero-blood vessels during diastole.|
Abnormal uterine artery Doppler in a pregnant woman: persistent diastolic notch
The early diastole notch remains potentially due to the vessels not enlarging or dilating as they should. This means that there is lower blood flow and less nutrient-rich blood for the developing baby.
We call this utero-placental vascular insufficiency or UPVI and it implies a restricted blood supply. (See abstract, article by Dr Kingdom and Dr Burton, published in Placenta, June 2009)
This also poses a significant risk factor for long-term maternal cardiovascular disease. Women who develop pre-eclampsia, IUGR, and delivery <32 weeks require follow-up of their own health, even with their blood pressure returns to normal. (See abstract, original article by Dr Yinon, published in Circulation, November 2010), and additional evidence).
The medical research literature on screening for placental insufficiency is dominated 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 test information (the maternal biochemistry) and when combined with both placenta morphology and uterine artery Doppler, we can assign a risk for the range of perinatal problems attributable to placental insufficiency rather than focusing on just one aspect. (See abstract, article by Dr Toal, published in the American Journal of Obstetrics and Gynecology, April 2007).
Summary of Placenta 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. In this review, only two blood tests (elevated AFP for IUGR and stillbirth; 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 these tests in their original MoM values, as opposed to merely looking at the bottom line risks of neural tube defect or Down syndrome.
However, to date, the data for PAPP-A is more limited, as the test was only introduced on a wide scale to screen for Down syndrome since approximately 2004 in North America. PAPP-A has been a recent significant research focus in our clinic. We recently reported our research in 90 women with very low PAPP-A when the developing baby did not have Down syndrome. Approximately 1 in 4 women were found on ultrasound to have a small, thick and abnormally-shaped placenta. This finding was associated with the subsequent risks listed under placental complications. 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.