6 Things You Didn’t Know About Your Placenta
Placentas are great! They are bloody and awkwardly meaty, and we’re not really used to seeing organs outside our bodies, but they are the key to sustaining babies as they grow during pregnancy and I find them incredible. Here are five facts about your placenta that –just might—help you find it as remarkable as I do.
It isn’t actually yours.
Your baby started out as just two bitty cells that merged together into one. It copied itself over and over and then the copies started to transform into different cell types with specific jobs (this is called differentiation). Some of those cells became trophoblasts, which eventually became the placenta. Around day 6 after fertilization, the tiny bundle of cells surrounded by a layer of trophoblasts snuggles down into the nutrient-rich lining of the uterus and begin their 40-ish-week relationship with mom’s cells. So, the placenta isn’t made out of mom’s genetic material, it’s made of baby’s genetic material.
It doesn’t allow mixing of blood
The placenta is the baby’s interface with mom. Through the placenta, mom provides nutrients and oxygen and takes out the “trash” of carbon dioxide and metabolic wastes. However, mom’s blood doesn’t come into direct contact with baby’s. Instead, the edge of the placenta looks like little finger-like projections (called villi) suspended in open spaces where mom’s blood pools. Each “finger” contains blood vessels from baby and its border is made of trophoblasts; these blood vessels are like the tiny twigs of a tree, and if you follow them back along “branches” to the center of the tree, they twine into the three vessel trunk of the umbilical cord. Baby’s blood can then exchange all the necessary materials (nutrients, gasses, wastes) with mom’s blood across the wall of trophoblasts without ever touching.
It is baby’s protector
The wall of trophoblasts is very selective about what it lets pass through. Sugar, oxygen, and carbon dioxide of course get a free pass. The wall turns back most harmful microbes (a wily few can work their way through and these are the dreaded TORCH infections). Even the mother’s immune system is held at bay—since the baby is not “self” to the mother, it otherwise wouldn’t be tolerated within the body and would be attacked by mom’s immune system. In an interesting twist, the placenta does permit some of mom’s antibodies to pass through during the third trimester. Baby’s immune system is naïve at birth, meaning it hasn’t been exposed to the environment yet, which means it can’t perform to its full potential. The temporary passive immunity from mom provided via the placenta (IgG) and breastmilk (IgA) supplements baby’s baseline immune function until their specific/adaptive immune system matures.
It produces hormones that sustain the pregnancy and modify mom’s metabolism
Women’s sex hormones (estrogen and progesterone) are primarily produced in the ovaries…most of the time. The ovaries provide the estrogen and progesterone needed for pregnancy up until the end of the first trimester, at which point the placenta takes over. In addition to estrogen and progesterone, the placenta produces the human chorionic gonadotrophin (hCG) that is detected in pregnancy tests, and human placental lactogen (hPL), which modifies mom’s metabolism to shunt blood glucose to the baby, as well as many others!
At birth, up to a third of baby’s blood is still in the placenta
This blood returns to the baby over time as the baby transitions to life on the outside. If the umbilical cord is clamped too soon, much of the baby’s blood is trapped in the placenta and never makes it back to nourish the baby. Babies who need help transitioning do better if they stay attached to the placenta, because they can rely on it as a reservoir of oxygenated blood as they bring their lungs online.
Placental separation is a mechanical process.
During pregnancy, the placenta is attached to the wall of the uterus and grows along with it as it stretches to accommodate the growing baby. Pretty quickly after the baby is born, the uterus collapses on itself like a balloon with the air let out. Meanwhile, the placenta remains about the size of a dinner plate. As a result, the placenta is sheared away as the surface area of the uterine wall dramatically decreases. The placenta is born within a few minutes and the uterus begins the work of contracting down, its muscle fibers clamping off all the many blood vessels that supplied the baby through pregnancy. At the end of pregnancy the placenta is the size of a dinner plate and occupies a spot rich with blood vessels. Midwives explain to moms that they after the placenta separates, they essentially have a large wound inside their uterus that needs to heal. This helps folks understand why it’s so important to take it easy in the first weeks after having a baby.