The placenta is a temporary organ that develops during pregnancy, acting as the life support system for the fetus. It primarily facilitates the exchange of oxygen and nutrients from the mother’s bloodstream to the baby. It also removes metabolic waste products, such as carbon dioxide, and produces hormones necessary to maintain the pregnancy. Understanding when this organ naturally concludes its work, or fails prematurely, is important for managing gestation.
Natural Decline: When the Placenta Slows Down
The placenta is designed to have a limited lifespan, functioning optimally for the typical 40-week human gestation. Its decline is not abrupt but a gradual process known as senescence, which is the natural aging of the organ. This aging process involves specific structural changes that begin to appear in the late third trimester, generally around the 36-week mark.
One common change is the accumulation of calcification, appearing as deposits within the placental tissue. While some calcification is a normal sign of maturity, it reflects a reduction in the surface area available for efficient exchange. At a cellular level, senescence includes the up-regulation of specific protein markers, such as p21 and p53, associated with cellular aging and programmed cell death.
As the pregnancy extends past the expected due date, particularly beyond 41 weeks, this natural decline becomes a greater concern. The efficiency of nutrient and oxygen transfer decreases, increasing the risk of complications for the fetus. For this reason, obstetric guidelines recommend increased fetal surveillance or delivery intervention when a pregnancy progresses to 41 to 42 weeks, as placental function becomes less predictable.
Recognizing Early or Pathological Failure
While natural decline is slow and linked to gestational age, the placenta can stop working effectively much earlier due to pathological failure, often termed placental insufficiency. This is not a matter of aging but a failure to properly develop or sustain adequate function, leading to a reduced supply of oxygen and nutrients to the fetus. This failure is often rooted in issues with blood flow and the development of maternal blood vessels feeding the placenta.
Several underlying medical conditions in the mother can accelerate this failure by compromising the blood supply to the uterus. Risk factors include chronic maternal hypertension, preeclampsia (characterized by high blood pressure and organ damage), and pre-existing or gestational diabetes. Lifestyle factors, such as chronic smoking or substance use, also negatively impact the placenta’s ability to develop a healthy vascular system.
The most significant consequence of premature placental failure for the fetus is Intrauterine Growth Restriction (IUGR), where the baby does not grow as expected due to inadequate nutritional supply. The fetus may also experience chronic lack of oxygen (hypoxemia) and show signs of stress. Placental insufficiency can lead to oligohydramnios, an abnormally low volume of amniotic fluid, as the fetus redirects blood flow away from the kidneys to protect the brain.
Monitoring and Managing Placental Health
When there is concern about placental function due to risk factors or a post-term pregnancy, medical professionals use specific tools to assess the baby’s well-being. One common method is the Non-Stress Test (NST), which monitors the fetal heart rate’s response to movement. A reactive result, showing accelerations in heart rate, is reassuring that the fetus is well-oxygenated.
A more comprehensive assessment is the Biophysical Profile (BPP), which combines the NST with an ultrasound evaluation of four specific physical parameters:
- Fetal breathing movements
- Gross body movements
- Muscle tone
- The volume of amniotic fluid
The BPP score provides a clearer picture of the fetus’s current health status and the long-term functioning of the placenta.
Another specialized tool is the Doppler ultrasound flow study, which measures blood flow resistance in the umbilical artery and other fetal vessels. Elevated resistance in the umbilical artery suggests that the blood vessels in the placenta are constricted, which indicates compromised placental function. When monitoring tests indicate that the placenta is no longer providing a safe environment, the primary intervention is timely delivery, often through induction of labor or a C-section, to prevent further fetal compromise.