Final answer:
The longest period of prenatal development is the fetal period, which begins at the ninth week after fertilization and continues until birth, lasting approximately 30 weeks.
Step-by-step explanation:
The longest period of prenatal development is the fetal period.
The fetal stage of prenatal development begins at the ninth week after fertilization and continues until birth. This stage lasts approximately 30 weeks and involves rapid growth and maturation of the organs and body systems. During the fetal period, the developing baby grows significantly in size, the organs further differentiate and become functional, and the features of the baby become more defined.
Prior to the fetal stage, a developing human is called an embryo from weeks 3 to 8 of gestation, which is when all organ systems are structured in rudimentary form. Before that is the pre-embryonic stage, which covers the first two weeks of development post-fertilization as the zygote undergoes early cell divisions until implantation in the uterus.