Final answer:
The mechanism that normally prevents multiple pregnancies with embryos at different growth stages is the prevention of polyspermy. Once a sperm fuses with an oocyte, mechanisms are activated to prevent further sperm from penetrating, ensuring that the embryo has the correct number of chromosomes.
Step-by-step explanation:
Prevention of Polyspermy in Human Embryos
An extremely rare occurrence in humans is a multiple pregnancy with embryos at different stages due to fertilization weeks apart. Normally, mechanisms are in place to prevent such events from happening. When the first sperm fuses with the oocyte, two mechanisms are deployed to prevent polyspermy: a fast-reaction that alters the electric potential of the membrane, and a slower response that releases enzymes, creating a hard layer around the oocyte, thereby stopping further sperm from penetrating.
This prevention is essential because if polyspermy were to occur, the resulting zygote would contain more than the standard two sets of chromosomes, creating a triploid organism. Such an organism is not genetically viable and tends to die within a few days. Therefore, the physiological blockades established by the oocyte are critical for ensuring the survival and proper development of the embryo.
Detailed embryonic development progresses through cleavage, blastulation, and implantation, with each stage closely regulated to avoid anomalies. Following successful fertilization, the zygote undergoes several rapid mitotic cell divisions, known as cleavage, while traveling towards the uterus, which does not increase the total volume of the conceptus, consisting of the zygote and its associated membranes. If the initial mechanisms to prevent additional sperm penetration fail, a nonviable embryo may result, leading to the cessation of the pregnancy at a very early stage.