Maintenance of Pregnancy and Embryonic Development

Once the blastocyst has implanted, pregnancy has to be actively maintained, and the embryo has to be built up, tissue by tissue, over the roughly 280 days of human gestation. This section works through that whole process: how the three primary germ layers are first laid down and what body systems each one eventually forms, the extra-embryonic support structures (including the placenta) that keep the embryo alive and connected to the mother throughout, the hormones the placenta itself produces to sustain the pr …