Which of the following is NOT a stage of human development in the womb?

The correct answer is the blastocyst. In human development, the initial stage is the zygote, which forms after the sperm and egg unite. Following fertilization, the zygote undergoes several divisions and develops into a blastocyst, which is an early stage that implants in the uterine wall.

However, blastocyst is indeed a transitional phase in early human development and is part of the sequence leading to the embryo stage. Once implantation occurs, the development continues through the embryo stage, which lasts until the end of the eighth week of pregnancy. After this period, the developing human is referred to as a fetus, which continues to grow and develop until birth.

It's important to understand the chronological sequence: zygote to blastocyst to embryo to fetus. In this way, while the blastocyst is an early and critical stage of development, it is often not classified as a distinct stage in the same way as the embryo and fetus are, especially when referring to typical terminologies in human development. This might lead to the confusion that categorizes the blastocyst as being less significant as a label compared to the other stages.