The developing fetus is maintained by the placenta, which develops from fetal and maternal tissues

The placenta consists of a fetal portion, formed by the chorion, and a maternal portion, formed by the decidua basalis. The two parts are involved in physiologic exchange of substances between the maternal and fetal circulation.

The uteroplacental circulatory system begins to develop around day 9, with development of vascular spaces called trophoblastic lacunae within the syncytiotrophoblast. Maternal sinusoids, which develop from capillaries of the maternal side, anastomose with the trophoblastic lacunae (Fig. 22.24). The differential pressure between the arterial

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