The placenta is an indispensable but temporary organ that physiologically joins the mother and the developing fetus. This remarkable, shared structure is the centerpiece of the complex dance that takes place between the needs of the mother’s body and the demands of the growing fetus. The placenta’s role is to facilitate the constant exchange of nutrients and wastes, including gases, as well as hormones and key immune factors.

The formidable challenge the placenta must first surmount, however, is rejection. Our immune systems are designed to identify and then reject anything that our bodies consider “foreign.” A growing fetus that has received half of its genetic material from its father qualifies as biologically foreign and should accordingly trigger a pregnant woman’s immune systems into swift and decisive action. But obviously rejection is not in the interest of the individual fetus, let alone the human species. So why doesn’t the mother’s immune system reject her developing baby?

Scientists do not yet know all the details of how this feat is accomplished, but it is clear that the survival of the embryo is an immunological balancing act that requires careful cooperation. (It’s thought that many spontaneous abortions are likely due to defects in this immunological compromise.) Initially, paternal proteins in the zygote are somehow “hidden” from detection after the egg is fertilized. In this way, the woman’s immune system is tricked into accepting the embryo.

The Powerful Placenta
As the fetus grows, the ongoing threat of rejection is avoided thanks to a strict separation of maternal and fetal blood supplies. This is accomplished by the placenta, which allows maternal and fetal capillaries to intertwine closely enough to allow the exchange of gas, nutrient, and messenger molecules, but keeps them separate enough to prevent the triggering of an immune response. “The placenta serves as the mediator of information, energy, and molecules between mother and baby,” explains Deepak Chopra. “And yet, the circulation of the baby and the circulation of the mother never actually mix. If they did, it would create chaos and severe immune consequences, both for the baby and for the mother.”

The placenta serves as a traffic cop, making sure that nutrients are delivered to the fetus and wastes removed, but doing its best to keep harmful substances out. The job is easier said than done, however. The reality is that certain pathogens, such as the measles virus, and poisons such as heavy metals, drugs, and alcohol do seep through to the fetus, and can have serious effects on normal growth and development. Studies have shown that even air pollution can harm a developing fetus. In many cases, the timing of the exposure plays a key role in the degree of impact.

The placenta mediates the biological conversation between mother and fetus, but that doesn’t mean it is a neutral party: its priority is clearly the developing fetus. Even in the early stages of implantation, the placental cells are extremely aggressive, breaching the uterine wall and behaving similarly to tumor cells. “Obstetricians have long referred to the placenta,” notes Sarah Blaffer Hrdy, in Mother Nature, “as ‘a ruthless parasitic organ existing solely for the maintenance and protection of the fetus, perhaps too often to the disregard of the maternal organism.’” Strong talk. Another motif historically invoked by medical researchers in describing the placenta, says Hrdy, was the language of warfare, borders, and defenses—in short, cellular combat between maternal and fetal tissues.

Whatever the excesses of language, however, the fact remains that the placenta necessarily wields great power and control over pregnancy. The placenta creates and maintains supply lines for incoming nutrients. Because the fetus needs glucose, the placenta produces a hormone, human placental lactogen, which increases glucose levels in the mother’s bloodstream. In response, the mother’s body produces more insulin to counteract the effects of high blood sugar. It is a balancing act between needs that can sometimes cause problems, which may be felt first by the mother, but then, of course, also impact the fetus.

By 7 weeks the placenta takes over the job of producing the hormone progesterone, which is vital for maintaining the pregnancy. The fingerlike villi that increase the absorptive surface of the placenta would stretch about 30 miles if fully extended. Until 15-16 weeks, the placenta is actually larger than the developing fetus. A powerful ally indeed! So important is the placenta to pregnancy and childbirth that many traditional cultures have adopted rituals surrounding its disposal; many of the rituals involve ceremonially burying the placenta.

“The umbilical cord,” says Chopra, “is a marvel of engineering.” It starts to develop at 42 days and is well established by 8-9 weeks. Two arteries and a vein are housed within the cord, which is structurally supported by smooth muscle and filled with a gelatinous substance known as Wharton’s jelly. The cord reaches its maximum length by 30 weeks and averages around 2 ft (60 cm). Because the umbilical vessels are longer than the cord, they must spiral within it. The cord itself is also somewhat coiled, and “this prevents it from being compressed or entangled,” explains Chopra.

Marvelous engineering seems a particularly apt phrase with which to describe the umbilical cord. It is perhaps no coincidence, then, that the advent of sophisticated medical imaging has moved scifi writers and NASA engineers alike to describe space walks in terms of floating astronauts tethered to their mother ships by umbilical lifelines.

More on this topic

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Joyous Mom, Joyous Baby
Your Baby Enters the World
Nourishing Body & Bond
Mother & Teacher
Nurturing Development
A Mutual Gaze
From Bump to Bundle

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