The Anatomy of the Placenta

The placenta develops within the uterus during pregnancy, playing a key role in nourishing and providing oxygen to the fetus, as well as removing waste material. This organ is attached to the wall of the uterus, with the baby’s umbilical cord arising from it. Throughout the course of a pregnancy, the placenta grows and changes shape, with its thickness being a reliable measure of how far along the mother-to-be is in gestation. Furthermore, a number of disorders can impact this organ, including placenta previa, in which some or all of the cervix is covered by the placenta, as well as placenta accreta malformations, which involve different degrees of implantation within the uterine wall.

Anatomy

Structure and Location

The largest fetal organ, the placenta undergoes rapid development over the course of pregnancy. By the time the baby is brought to term, it has a flat, round disc-like shape that is about 22 centimeters (cm) in diameter, with walls that are typically between 2 and 2.5 cm.

The placenta typically sits along the back wall of the uterine wall—about 6 cm from the cervix—occasionally accessing the side walls throughout its course of development. Significantly, the umbilical cord (which brings in nutrients and oxygen and takes out waste material) connects the mid-section of the fetus to the placenta; in turn, the fetus is surrounded by the amniotic or gestational sac.

The placenta undergoes consistent change throughout the course of pregnancy; between week 0 and 13 after conception, the fertilized blastocyst (what the embryo becomes once its cells start differentiating at about five days after the egg is fertilized) embeds itself in the mucous membrane (endometrium) of the uterine wall, allowing for the fetus and placenta to start forming. By the fourth or fifth month of pregnancy, the placenta takes up about half of the uterine surface, though this percentage shrinks as the fetus grows. At birth, the placenta is also ejected from the body.

Crucial to placenta (and, by extension, embryonic) development is the formation of small, finger-like structures called chorionic villi, which are composed of two types of cells—cytotrophoblasts and syncytiotrophoblasts. The former of these interact with arteries and veins in the walls of the uterus to ensure the fetus gets the nutrients and oxygen it needs. Throughout pregnancy, this vasculature grows in size and complexity, allowing for the formation of the following two major components.

• Maternal component: Essentially, this is the portion of the placenta that is formed of the mother’s endometrium or the maternal uterine tissue. It forms what is called the decidua basalis, or maternal placenta.
• Fetal component: Also known as the chorion frondosum or villous chorion, this is the portion of the placenta arising from the blastocyte.

These are held together by outgrowths, called anchoring villi, from the maternal component. The placenta is surrounded by a placental membrane or barrier. While it serves to differentiate blood supply for mother and fetus, many substances can still get through.

Anatomical Variations

Not every placenta forms regularly, and this can have serious implications. Several such malformations, including placenta previa, accreta, increta, and percreta, are considered serious medical conditions that can endanger a mother, the fetus, or both. In addition, there are a number of other commonly identified abnormalities.

• Bilobed placenta: Also known as “placenta duplex,” this is a case where the placenta is composed of two roughly equal-sized lobes. The umbilical cord may insert into either lobe, run through both, or sit between them. Though this condition doesn’t increase risk of damage to the fetus, it can cause first-trimester bleeding, excessive amniotic fluid within the gestational sac, abruption (premature separation of the placenta from the womb), or retained placenta (when the placenta remains in the body after birth). This condition is seen in 2% to 8% of women. 
• Succenturiate placenta: In these cases, a lobe of placenta forms separately from a main body that is linked via the umbilical cord to the fetus. Essentially, it’s a variation of a bilobed placenta that occurs more commonly in women who are of advanced maternal age or in those who have had in vitro fertilization. Seen about 5% of the time, this condition can also lead to retained placenta as well as placenta previa, among other complications. 
• Circumvallate placenta: This is when the membranes of the placenta tuck back around its edges to form a ring-like (annular) shape. In this case, the outer membrane, known as the chorion causes a hematoma (a collection of blood) at the margin of the placenta, and vessels within its ring stop abruptly. This condition can lead to poor outcomes for the pregnancy due to the risk of vaginal bleeding during the first trimester, potential rupture of the membranes, pre-term delivery, insufficient development of the placenta, as well as abruption. This condition isn’t easily diagnosed during pregnancy.  
• Circummarginate placenta: This is a much less problematic variant of the above, in which the membranes do not curl back.
• Placenta membranacea: In this rare condition, chorionic villi cover the fetal membrane partially or completely, causing the placenta to develop as a thinner structure at the periphery of the membrane that encloses the chorion. This then leads to vaginal bleeding in the second and/or third trimester of pregnancy and may lead to placenta previa or accreta. 
• Ring-shaped placenta: A variation of placenta membranacea, this condition causes the placenta to have either a ring-like or horseshoe-like shape. Occurring in only about 1 in 6,000 pregnancies, this leads to bleeding before or after delivery, as well as reduced growth of the fetus.
• Placenta fenestrata: This condition is characterized by the absence of the central portion of the placenta. Also very rare, the primary concern for doctors is retained placenta at delivery.
• Battledore placenta: Sometimes called “marginal cord insertion,” this is when the umbilical cord runs through the margin of the placenta rather than the center. This occurs in between 7% and 9% of single pregnancies, but is much more common when there are twins, happening between 24% and 33% of the time. This can lead to early (preterm) labor and problems with the fetus, as well as low birth weight.

Function

The placenta plays an absolutely crucial and essential role during the nine months of pregnancy. Via the umbilical cord and the chorionic villi, this organ delivers blood, nutrients, and oxygen to the developing fetus. In addition, it works to remove waste materials and carbon dioxide. As it does so, it creates a differentiation between maternal and fetal blood supply, keeping these separate via its membrane.

Furthermore, the placenta works to protect the fetus from certain diseases and bacterial infections and helps with the development of the baby’s immune system. This organ also secretes hormones—such as human chorionic gonadotropin, human placenta lactogen, and estrogen—necessary to influence the course of pregnancy and fetal growth and metabolism, as well as labor itself.

Associated Conditions

Aside from the developmental abnormalities listed above, the placenta may also be subject to a number of medical conditions that may be of concern to doctors. Oftentimes, the core of the problem has to do with the position of this organ. Among these are the following.

• Placenta previa: This condition occurs when the placenta forms partially or totally toward the lower end of the uterus, including the cervix, rather than closer to its upper part. In cases of complete previa, the internal os—that is, the opening from the uterus to the vagina—is completely covered by the placenta. Occurring in about 1 in 200 to 250 pregnancies, risk factors for placenta previa include a history of smoking, prior cesarean delivery, abortion, other surgery of the uterus, and older maternal age, among others. Depending on the case, cesarean delivery may be required.   
• Placenta accreta: When the placenta develops too deep within the uterine wall without penetrating the uterine muscle (myometrium), the third trimester of the pregnancy can be impacted. A relatively rare occurrence—this is the case in only 1 in every 2,500 pregnancies—this condition is more likely to occur among smokers and those with older maternal age, as well as those with a history of previous surgeries or cesarean deliveries. This also can happen alongside placenta previa. During delivery, this condition can lead to serious complications, including hemorrhage and shock. While hysterectomy—the removal of a woman’s uterus—has been the traditional treatment approach, other, more conservative options are available.     
• Placenta increta: Representing 15% to 17% of placenta accreta cases, this form of the condition is when development of the placenta is within the uterine wall and it penetrates the myometrium. Childbirth is severely impacted in these cases, since this can lead to severe hemorrhage due to retention of the placenta within the body. As such, cesarean delivery is required alongside hysterectomy or comparable treatment.   
• Placenta percreta: Yet another type of accreta, placenta percreta occurs when this organ develops all the way through the uterine wall. It may even start to grow into surrounding organs, such as the bladder or colon. Occurring in 5% of placenta accreta cases, as with placenta increta, cesarean delivery and/or hysterectomy is necessary in these cases.
• Placental insufficiency: Arising for a range of reasons, this is when the placenta is unable to provide enough nourishment for the fetus. This can be due to genetic defects, deficiencies of vitamins C and E, chronic infections (such as malaria), high blood pressure, diabetes, anemia, or heart disease, as well as other health issues. Treatment can range from ensuring better diet to taking medications like low-dose aspirin, heparin, or sildenafil citrate.

Tests

Throughout the course of pregnancy, doctors will perform a wide range of tests to ensure the health of the fetus. This can mean everything from blood tests to genetic tests are administered. When it comes to ensuring proper development of the placenta, a number of diagnostic techniques are employed, including the following.

• Ultrasound: A frequently employed approach when it comes to monitoring fetal development as well as the health of the placenta, ultrasound employs high-frequency sound waves to create a real-time video of the uterus and surrounding regions. Especially in the second and third trimesters, this approach can be used for cases of placenta previa, among other disorders. Furthermore, based on ultrasound results, doctors classify placental maturity. This system of placental grading ranges from grade 0 for pregnancy at 18 or less weeks to grade III for when things have progressed beyond week 39. Early onset of grade III, for instance, may be a sign of placental insufficiency.
• Chorionic villus sampling (CVS): A good way to perform genetic testing, CVS involves taking a small sample of the placenta using a specialized catheter (tube) that is inserted through the vagina and cervix using ultrasound as a guide. This can also be done using a syringe and going through the abdominal muscles. The sample is then sent to the lab for testing, with results available between seven and 10 days.
• Magnetic resonance imaging (MRI): This imaging approach relies on strong magnetic and radio waves to create highly detailed depictions of the fetus and placenta. Though not necessarily the first line of treatment, MRI may be used to diagnose placenta increta and percreta. In addition, this method may be used in cases of placental insufficiency.