The Amazing Placenta

By Sarah J. Buckley, MD
Issue 131, July/August 2005

Jacob's conception was unexpected, and unknown to us for several weeks. We'd been on holiday in Tasmania, Australia's small "south island," and on the ferry trip home had carried not only Emma (four years) and Zoe (one year), but also their brother-to-be—a tiny mass of cells barely a week from conception. As I slept fitfully on my bunk, Jacob's blastocyst, looking like a tiny blackberry just 2 millimeters in diameter, had already rolled down one of my fallopian tubes and was busy burrowing into the dark, thick lining of my womb.

For the next two weeks, Jacob-to-be obtained his nourishment directly from this rich lining, and I was oblivious to his presence. Quiet he may have been, but he was not quiescent—it was during this important time that Jacob's cells first became specialized, and he began to form his placenta. Deep inside his blackberry-shaped blastocyst, some of his cells clumped together to form an "inner cell mass" that would later become Jacob's body, umbilical cord, and amniotic sac. Other cells migrated outward to form the surrounding trophoblast, which would become Jacob's placenta.

Once created, his trophoblast began infiltrating my womb more deeply, releasing enzymes to dissolve my uterine cells and blood vessels. In this way, Jacob created lakes of my blood—the placental lacunae—for his sustenance. Even as I attributed my overdue period to intensely breastfeeding Zoe while on holiday, Jacob's villi—fingerlike projections from his developing placenta, each containing a newly formed blood vessel—were growing and dipping into my lacunae, our bloodstreams separated by the thinnest, most permeable of membranes. (1)

Through this membrane, for the rest of the pregnancy, I would pass on all the nutrients and growth factors that Jacob's body needed, and he would pass his wastes back to me. Furthermore, this membrane would prevent our blood cells from mixing, and my immune system from rejecting Jacob as a foreign invader.

As well as this, Jacob's villi were anchoring his developing placenta, acting as his roots in the firm soil of my womb-garden, and his body stalk—the tissue that would later become his umbilical cord—was keeping his embryonic body alive and attached to his placenta, like a floating astronaut's lifeline. At this time his body was smaller than a kidney bean, and just beginning to form limb buds—his future arms and legs.

Jacob's developing placenta had another important early task, the production of placental hormones, and it was this that revealed his presence. Under the influence of human chorionic gonadotropin (HCG), which his placenta had been producing in increasing amounts since a few days post-implantation, I was beginning to feel decidedly queasy. I finally realized that I was pregnant when this nausea visited me in the middle of the night. HCG was also the hormone that turned my pregnancy test predictably positive the next day.

Over the next few weeks I had intense, all-day morning sickness. Maybe it would have been more tolerable had I realized that this shift, which put me off spicy and bitter foods as well as tea and coffee, was actually caused by Jacob's placental hormones working to protect him from the high levels of natural toxins that such foods contain. Furthermore, my heightened sense of smell—another trigger for nausea—was ensuring that I ate only the freshest foods and avoided pungent aromas and cooking vapors, which could also contain inhalable toxins. My nausea began to subside as Jacob grew beyond the embryonic stage (about eight weeks after conception) and was almost gone by the fourth month of pregnancy, when his organ systems were essentially fully formed and therefore less vulnerable to toxic damage. (2)

This two-month milestone (equivalent to 10 weeks from menstruation) marked the beginning of Jacob's life as a fetus. By this time, his body had reached 4 to 5 centimeters in length, and thanks to the nourishment delivered by his placenta, his weight had increased to a creditable 4 grams, or one-seventh of an ounce—220,000 times greater than his weight at conception. The trophoblast that had originally surrounded him had by now formed a near-mature placenta on one side and, on the other, a protective bubble, the chorion, that would eventually form part of Jacob's double-layered membranes.

Over the next two months, Jacob's placenta grew and spread. By mid-pregnancy, his placenta covered about half the wall of my uterus and was heavier than his body. Later in the pregnancy, Jacob's body would grow much more, so that at birth his placenta would weigh about one-sixth as much as his body. Jacob's versatile placenta was also able to migrate during pregnancy, moving slowly toward the best blood supply and away from areas of diminished supply. This mechanism, known as trophotropism, is thought to explain many irregularities in placental shape and structure, as well as the healthy upward movement of most placentas that are low-lying (placenta previa) in early pregnancy. (3)

Although external to his body, Jacob's placenta was his most essential organ, performing all the functions that his immature gut, lungs, immune system, kidneys, liver, and skin were not capable of in my womb.