Final answer:
Emily and her husband are observing their baby during the fetal stage of development, which begins at the ninth week after fertilization and lasts until birth. This stage follows the embryonic stage where initial organ formation occurs and is characterized by significant growth and maturation of the fetus in preparation for birth.
Step-by-step explanation:
When observing an ultrasound of their developing baby, Emily and her husband are viewing the baby during the fetal stage. This stage begins at the ninth week after fertilization and continues until birth. The fetal stage is marked by significant growth, development, and maturation of all organ systems, which began during the preceding embryonic stage. In contrast, the embryonic stage lasts from approximately the third to the eighth week following fertilization, a time when the foundational structures of the body are formed and the basic organ systems begin to develop.
During the initial two weeks post-fertilization, we have the pre-embryonic stage, where the conceptus (the zygote and its surrounding membranes) undergoes rapid cell divisions without an increase in the overall volume. This pre-embryonic stage is followed by the embryonic period, where the embryo implants in the uterus and the organ systems start forming in rudimentary form. Subsequently, the fetal stage encompasses the extensive growth and maturation needed for postnatal life.
Every prenatal stage is distinct and critical for the development of the fetus, and the transition from an embryo to a fetus signifies that the basic structures are already in place and now the baby is undergoing extensive growth and preparing for birth.