Commentary
The importance of studying the placenta
The placenta has been extraordinarily poorly understood. Much of medicine has regarded the placenta like the appendix – an organ that may be...
One take-away from both of these studies has been that increased variability of placental shape is associated with lower placental functional efficiency. Moreover, in the University of North Carolina cohort, the impact of placental vascular pathology (either maternal uteroplacental or fetoplacental) on placental efficiency and function was shown to be dependent on shape. Only in the case of irregularly shaped chorionic plates did each of the two pathologies have a significant association with placental inefficiency.3
The realization that placental size (weight/mass/volume) may serve as a proxy for the fetoplacental basal metabolic rate came after it was shown that Kleiber’s law, which states that basal metabolic rate (BMR) is proportional to the body mass to the 3/4 power, can be applied to the newborn’s birth weight by substituting placental weight for BMR.
This fetal-placental version (placenta weight = .75 birth weight) of Kleiber’s law was validated through an analysis of the sets of placental measures and birth weights stored in the Collaborative Perinatal Project. It has implications for our ability to use ultrasound and Doppler measures to predict risk and to understand pathologic pregnancies, such as those complicated by diabetes or fetal growth restriction.
Courtesy Dr. Carolyn M. Salafia
In this irregular placenta, there is no consistent pattern to the shape but the placenta has grown where it could, creating the "best" placenta possible for the fetus.
Research also has shed light on the timing of shape variants. We now know that abnormalities of placental surface shape result mainly from early influences – perturbations of placental growth that occur no later than mid-gestation – rather than from trophotropism (the placenta “grows where it can and does not grow where it can’t”) and passive uterine remodeling later in pregnancy, as has traditionally been believed.4
With respect to the umbilical cord, the location of cord insertion is independent of eventual disk shape, but is to a large degree determined by the end of the first trimester. In addition, cord insertion does influence and is correlated with chorionic vascular density and with disk thickness. Greater eccentricity of cord insertion appears to be linked to increased placental disk thickness, each of which is independently associated with reduced placental functional efficiency.5,6
We have worked with placentas from newborns in families with an older child diagnosed with autism and have found significant differences between these placentas and the placentas of low-risk newborns. In particular, we have measured a reduction in the number or chorionic surface vessel branch points of more than 40%.
Current implications
Irregularities in placental surface shape, disk thickness, and various descriptors of placental size may all be determined from ultrasound and Doppler imaging. We can also assess cord insertion and chorionic surface vessel distribution, track patterns and rates of placental growth, and use various placental measures to understand placental efficiency and to improve the specificity of placental histopathologic diagnoses.
At this point, our use of in vivo imaging of the placenta has mainly involved grayscale ultrasound, but with color or power Doppler and improved surface network tracing protocols, we could save the red and blue areas we visualize as a “shape” and assess the density of surface vessel branching, for instance, and the degree of uniformity in vessel distribution.
Courtesy Dr. Carolyn M. Salafia
This visual shows traced placental chorionic surface vessels, with circles representing estimated chorionic disk areas at 10 and 2 weeks' gestation.
We currently have quantitative markers of placental shape and mathematical models to help us identify at-risk pregnancies. What we need are more data from early ultrasounds (from all pregnancies and not only complicated ones) and more comprehensive and precise models of placental growth and function. This will enable us to better identify preclinical fetoplacental pathophysiology and predict downstream risks.
In the meantime, the delivered placenta can be a valuable source of information – an extra dimension for looking back in time. With a paradigm shift toward more thorough pathologic analysis, the delivered placenta can provide unique insights into how placental growth evolved during the pregnancy.
Do not throw away the placenta, and do not just weigh it. Take a photograph, because even with a photograph we can assess vascular density, disk thickness, and other placental characteristics.
In the case of pregnancy complications or suboptimal outcomes, the knowledge we can gain from the delivered placenta can help the physician and patient to understand recurrence risks and to better target evaluation, monitoring, and management in the next pregnancy.
References