Activity During Oocyte Activation and the Three Trimesters of Gestation

Anatomy and Physiology
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What Activity Prevents Penetration by Additional Sperms
Successful fertilization requires an egg to be fertilized by a single sperm. The human body has mechanisms that protect eggs from penetration by other sperms (polyspermy). When an oocyte is fertilized, its plasma membrane depolarizes and sheds off the sperm receptor (folate receptor 4) (Bianchi & Wright, 2020). This process blocks the entry of more spermatozoa. The entrance of the first sperm triggers the release of intracellular calcium, initiating the process of oocyte activation. Frequent oscillation of intracellular calcium levels triggers exocytosis of cortical granules containing enzymes that alter the zona pellucida upon fertilization (Bianchi & Wright, 2020). The proteinous zona pellucida surrounding the oocyte becomes rigid, preventing further entry of spermatozoa.
Events that occur between Exposure of Oocyte to Spermatozoa and Formation of the First Blastomere Pair
Exposure of the oocyte to the spermatozoa results in the fusion of nuclei from the two cells, forming a zygote. The oocyte undergoes meiosis, where the number of chromosomes reduces by half. Homologous chromosomes part, forming haploid cells that contain one chromosome from each pair. The spermatozoa and the oocyte now have half as many chromosomes per cell. The union of the haploid nuclei from both cells forms a diploid cell called a zygote. The zygote undergoes mitosis and divides into two cells to create the first blastomere pair.
The Three Trimesters of Gestation
The first, second, and third trimesters comprise the three gestational trimesters. The first trimester lasts from conception to the end of 12 weeks, during which most organs and body structures begin to form. The pregnant woman may experience nausea, vomiting, and frequent urination. Most defects and abortions occur during this trimester.
The second trimester begins from 13 weeks to 26 weeks post-conception. Unpleasant pregnancy symptoms start to disappear, and fetal movements are detectable. The sex organs differentiate during the second trimester; an ultrasound can confirm the baby's gender. Pregnant women may experience heartburn, back pain, and leg cramps at this stage. 
The third and last trimester starts from 27 weeks to 40 weeks. The fetus' organs develop fully and increase in size during this trimester. Both the fetus and the pregnant woman significantly gain weight. The woman may experience shortness of breath, leg cramps, difficulty sleeping, and hemorrhoids. From the seventh month, the fetus's lungs expand in preparation for birth.
The Events of Embryonic Development from Conception to the Fetal Stage
Prenatal development is classified into germinal, embryonic, and fetal stages. The germinal stage begins when the oocyte and spermatoz...