The placenta is missing the door and the key the virus needs.
Can a mother pass Covid-19 on to her baby? Pregnant women are scared of Covid-19, and so are the medical providers who care for them. The question of mom-to-baby transmission keeps Obgyns awake at night.
When the coronavirus pandemic first hit, Obgyns around the world wanted to know if mothers could pass SARS-CoV-2 to their babies, a process called vertical transmission.
The vertical transmission of Covid-19 was an unthinkable nightmare putting humanity’s next generation of babies at risk.
Fortunately, data surveillance so far indicates in-utero coronavirus infections are very rare. According to the American College of Obgyn (ACOG) July practice advisory, “the data are reassuring that vertical transmission appears to be uncommon.”
Why are fetal coronavirus infections rare?
New NIH research may explain why pregnant women with SARS-CoV-2 pass the infection to their baby infrequently. Examining the placental response to the virus demonstrates the difference.
Research shows the placenta may be missing what the virus needs to enter the fetal circulation: the door and the key.
The placenta protects the baby from infection
The placenta is the organ inside the uterus that keeps the baby alive. Maternal blood circulates through this internal filtration system. The growing baby receives oxygen and nutrients from the mother, and the placenta removes carbon dioxide and waste products.
The placenta is also the defense system against invading infections like bacteria and viruses. Placental immune system cells called trophoblasts are the first line of defense against any invading organisms.
Most infectious invaders do not make it across the placental defense system. Trophoblasts stop them in their tracks. But babies are at risk of a group of maternal pathogens called TORCH infections.
These diseases have developed ways to bypass trophoblasts. They cross the maternal-fetal barrier and cause illness in the baby. The TORCH infections are:
- Other (like ZIKA)
How does the placenta protect against Covid-19?
For SARS-Cov-2 to enter the placenta it needs to find an entry point and use its spike protein to enter cells. The human placenta may be missing both the door and the key the virus needs to find its way inside the fetal circulation.
The door is the ACE2 receptor. SARS-CoV-2 enters cells through the ACE2 receptor located all over our bodies. These receptors are in our nose, lungs, heart, blood vessels, kidneys, liver, and gastrointestinal tract.
The placenta has very few ACE2 receptors. It lacks receptors because the human placenta is missing the molecules needed to create it. Without the entry portal, SARS-CoV-2 is unable to bypass the placental defense system and cause infection.
Because the placenta has a low-level ACE2 receptor expression, SARS-CoV-2 can not find the door to enter cells to cross over from mom to baby.
But the placenta does have a small level of ACE2 receptors. Even with a small number of receptors, coronavirus could still infect a fetus. Fortunately, the virus needs something more than just an entry point. The ACE2 receptor door is deadbolted and locked. SARS-CoV-2 needs a key to open it.
Researchers demonstrate the placenta is also missing the key the virus needs to unlock the door when it locates an ACE2 receptor.
The placenta tissues lack the mRNA needed to produce an enzyme called TMPRSS2. This enzyme primes the SARS-CoV-2 spike protein allowing it to enter cells. TMPRSS2 is the key and without this enzyme, the virus is unable to enter.
Both the ACE2 receptor and TMPRSS2 are present in the placenta but only in tiny amounts. The low levels likely indicate SARS-CoV-2 can’t find the door and is missing the key to enter into the fetal circulation.
Many questions remain regarding the risk of pregnancy and Covid-19. This research on the protective effects of the placenta is encouraging. At the same time, fetal infection can occur in rare cases.
In Dallas, Texas, Parkland Hospital reported one of the first verified cases of intrauterine Covid-19 infections in The Pediatric Infectious Disease Journal on July 10, 2020.
As research continues, let’s hope the placenta keeps protecting babies from harm.
Blog Author: Dr. Jeff Livingston
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