The placenta is an essential and universal part of human development — supporting health and growth not only during the fetal stage but also well into adulthood. Because of its significance, placental function during pregnancy and the role of the growing placenta have been studied extensively.
However, there is limited understanding of this uniquely temporary organ, as past research has predominately focused on the placenta after delivery, when it has effectively stopped functioning. More real-time research is emerging that provides a deeper level of insight into the role of the placenta in both disease and health.
The Main Functions of the Placenta
The main function of the placenta, according to The Human Placenta Project, is to bring oxygen and nutrients to the fetus, as well as to produce hormones that maintain a normal pregnancy and support fetal development. The placenta also regulates amniotic fluid levels, provides immune protection, removes waste from the fetus and prepares the parent's body for birth and lactation. Serving as the interface between parent and fetus, the placenta undergoes very rapid growth and differentiation. Understanding the importance of the placenta to fetal development and growth reveals how placental abnormalities can have significant implications for a fetus.
Problems that arise in the development and function of the placenta can lead to serious outcomes, such as miscarriage, preterm birth, fetal growth restriction or chromosomal abnormalities. Further, many drugs are able to cross the placenta and can have a harmful effect on the fetus during pregnancy.
There is also an entire field of study focusing on the fetal and infant origins of disease. A landmark study from 1995 published in the European Journal of Clinical Investigation first connected the in-utero environment during fetal development to serious health issues in adults, including stroke, diabetes, hypertension and coronary heart disease.
How the Placenta Develops Throughout Pregnancy
The human placenta begins to develop five to 10 days after fertilization, forming from the outer layer of the pre-implantation embryo (the blastocyst) as it implants in the uterine wall. The blastocyst is composed of two distinct cell types: the inner cell mass, which forms the fetus and fetal membranes, and the outer trophoblast cells, which form the placenta.
After implantation, the placenta forms tiny hair-like projections called placental villi that embed into the uterine wall and begin to reshape the spiral arteries within to create sources of maternal blood, oxygen and nutrients. The reshaping process enlarges the vessels and transforms blood flow into a low-resistance, high-velocity flow.
Throughout the pregnancy, the villi increase their branching into a complicated tree-like arrangement to maximize the area of contact with the spiral arteries and meet the increased demands of the developing fetus. If the spiral arteries are not optimally remodeled during this phase of development, then the abnormal placental function can lead to an abnormal pregnancy, reports the National Institutes of Health.
The placenta is fully formed by 18–20 weeks gestation and will continue to grow in size throughout the pregnancy. By the time of delivery, it should weigh approximately 1 pound.
Normal and Abnormal Placenta Appearance
In the first trimester, a normal placenta will appear on ultrasound as a diffuse, echogenic ring around the gestational sac, often referred to as the double decidual sign. At this stage, a sonographer should scan for a chorionic bump: an abnormal finding involving the developing placenta that can indicate a poor pregnancy outcome. This is typically an incidental finding that appears as a focal protrusion of the chorion layer of the placenta into the fluid-filled gestational sac. This is thought to represent an arterial hematoma and has been associated with an increased risk of miscarriage.
In the second trimester, a well-formed, normal placenta will be easy to visualize on ultrasound. It should appear uniformly echogenic and homogenous, with a vascular retroplacental area between the placenta and myometrium.
A third-trimester placenta contains more hypoechoic and anechoic areas and calcifications, making it appear more heterogeneous overall. Typically, calcifications that are seen after 30 weeks and located at the placental-myometrial area constitute a normal finding.
Generally, the thickness of the placenta will increase during the pregnancy with the thickness in millimeters corresponding to the gestational age in weeks. An abnormally thick placenta is associated with poor fetal outcomes. This can indicate the presence of macrosomia, infection, uncontrolled diabetes, fetal hydrops or chromosomal abnormalities.
Alternately, a very thin placenta can indicate a fetal growth restriction or aneuploidy. When a placenta appears small or thin, it indicates that the growth of the organ is restricted, with reduced weight and surface area. This affects the overall nutrition that a fetus has access to and often results in a low birth weight. Lower birth weight and small body size at birth can have lifelong consequences, such as increased risk for hypertension and blood pressure problems, reports research from the American Journal of Epidemiology.
Placental Abruption
Placental abruption occurs when part or all of a placenta separates from the uterine wall after 20 weeks gestation due to a rupture of vessels in the basal layer of the endometrium. Blood can accumulate in the retroplacental space between the placenta and uterine wall and lead to detachment and destruction of the placental tissues. A detached placenta is unable to function normally, which can quickly lead to fetal injury and even death.
Patients with placental abruption usually present with vaginal bleeding, pain, and a tense and painful uterine area on palpation. Sonographic assessment of placental abruption is a challenging emergency exam, but the use of both greyscale and color/power Doppler can be used to assess for areas of active bleeding or a large collection of blood in the placenta area.
The Placenta and Fetal Heart Abnormalities
The placenta and the fetal heart are two of the first organs to differentiate and develop during pregnancy. Micronutrients such as folate, along with several key genes, regulate the essential steps in the formation of both organs.
A study published in Frontiers in Physiology offers evidence that cardiac and placental development are interconnected and, therefore, so are their abnormalities. Specifically, defects in placental development, seen as reduced trophoblast invasion of the maternal spiral arteries and limited vascularization, may influence the development of the fetal heart and its function after birth.
How Ultrasound Helps Monitor Placental Growth
Ultrasound is used extensively in intrapartum examinations of the fetus and placenta. Routine ultrasound exams should occur at least twice in a normal pregnancy, the first between 7-14 weeks gestation and the second between 18-22 weeks. The placenta and fetus are routinely imaged at these times and examined for any signs of abnormalities. If any part of the placenta or fetus is not well visualized transabdominally, then a supplemental transvaginal ultrasound should be performed to better evaluate the area.
When assessing the placenta, the exact location must be documented, as well as the thickness and location relative to the cervix and umbilical cord. A sonographer should examine the connection of the placenta to the retroplacental complex and then measure and image any placental abnormalities. Intrapartum exams should devote special attention to the fetal heart in order to determine if there are any structural abnormalities present.
A growing body of research has shown how strong a connection there is between healthy placental function during pregnancy and a healthy fetus — as well as health in childhood and adulthood. A careful sonographic assessment can identify abnormalities that have a profound impact on the management of pregnancy and maternal and fetal health.
