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How does maternal blood reach the spaces around the placental villi?

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Close-up of a home pregnancy test device showing a blue plus sign in the result window, indicating a positive result for pregnancy. — pregnancy Devices for individual use at home detect pregnancy by measuring levels of the hormone human chorionic gonadotropin (hCG) in urine.

Maternal blood reaches the spaces surrounding the placental villi through the erosion of the endometrium and the underlying uterine blood vessels during the early stages of pregnancy.

As the trophoblast invades the uterine lining, it surrounds minute branches of the uterine arteries. The subsequent erosion of the endometrium around these blood sinuses allows them to open into small cavities, known as syncytial lacunae, within the trophoblast. These cavities eventually coalesce to form the intervillous spaces—a large, lakelike reservoir of maternal blood.1

Once the placenta is fully developed, maternal blood flows directly from the branches of the uterine arteries into this intervillous space, where it bathes the chorionic villi. The blood is then drained from this space by the uterine veins, maintaining a constant circulation that facilitates the exchange of nutrients, gases, and waste products between the maternal and fetal bloodstreams.12