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Scientists have discovered a previously unknown type of cell that appears only during pregnancy and may play a key role in how the placenta develops — though its exact function remains unclear.
The finding comes from a detailed atlas of the placenta and uterus, which maps how these organs grow and adapt to support a developing fetus, according to Live Science.
Researchers found that this new subtype of cells is absent outside of pregnancy but rapidly increases in number in its early stages, as the uterine lining transforms to support the embryo.
According to lead author Cheng Wang of the University of California, San Francisco, and senior researcher Jingjing Li, the discovery came as a surprise.
“We asked everyone — no one knows what this is,” Li said.
What the cells might do
Scientists believe these cells may help connect the placenta to the mother’s bloodstream and regulate how fetal cells invade the uterus — a critical process for establishing proper blood flow.
The cells also carry receptors that respond to cannabinoids — substances produced naturally by the body as well as compounds found in cannabis (such as THC and CBD). This could help explain why cannabis use during pregnancy is linked to health risks, including reduced blood flow to the placenta, lower oxygen supply to the fetus, premature birth, and low birth weight.
However, researchers caution that these cells alone likely do not fully explain those risks, and further study is needed.
Link to pregnancy complications
Using machine learning, scientists analyzed around 1.2 million cells and identified a connection between gene activity and cell behavior.
They found that the newly discovered cells help regulate how deeply fetal cells penetrate the uterine wall. If this process is disrupted — either too shallow or too deep — it can lead to complications such as preeclampsia or abnormal placental growth.
According to the researchers, these cells act as a kind of “brake,” sending signals that prevent the process from progressing too quickly — making them a potentially crucial factor in maintaining a healthy pregnancy.
