How does the placenta function?

Dr. Michael Roizen, MD
Internal Medicine
The placenta basically functions like this: Mom's blood flows into the blood lake on her side of the placenta, which bathes the chorionic villi that are threaded through with fetal blood vessels. Small molecules and nutrients (like oxygen, glucose, vitamins, fatty acids, calcium, antibodies, and so on) flow from Mom to baby, and waste products (like carbon dioxide, urine, and metabolic wastes) flow from baby to mom.

Weighing in at about 1 ¼ pounds when fully grown, the placenta works like a two-way filter. Stuff goes through it from one side to the other and vice versa. But the placenta is no Brita. It doesn't necessarily screen out the bad and only let the good pass through. The placenta lets everything through below a certain size and blocks insulin, heparin, and other large molecules that would otherwise cause immune rejection. That means any toxins that make the size cut can get passed to the fetus, whether it's gunk from cigarettes, saturated and trans fats, alcohol, or other nasty substances.

Your blood pressure forces blood around the villi so the villi can absorb and pass needed nutrients and antibodies for immunity to your baby, and transfer waste from the baby to your circulation for you to get rid of. During this process, there is no direct blood-to-blood contact between mother and child. About the same time that the placenta is forming, your hormones stimulate the inside layer of your uterus-already plush with blood vessels--to grow those blood vessels larger to be able to facilitate the exchange.

Once the placenta is fully formed and attached to the uterine wall, about the end of the first trimester, you can see a distinct difference between the maternal territory and the fetal territory. The maternal side is red and bumpy (from the shape of the villi poking through), while the fetal side is like a skating rink-smooth and slick, with the umbilical cord projecting from the surface.
YOU: Having a Baby: The Owner's Manual to a Happy and Healthy Pregnancy

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YOU: Having a Baby: The Owner's Manual to a Happy and Healthy Pregnancy

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.