The five domains of development in a pregnancy are fertilization, cleavage, blastocyst, implantation and fetal stages. These domains are collectively referred to as the period of prenatal development, which is the process in which the fetus or embryo gestates during pregnancy, starting from fertilization up until the time of birth. Careful observations performed through a series of prenatal check-ups during each stage are important to assess the baby’s overall health and normal development.
Fertilization is the stage when the sperm fertilizes the ovum or oocyte (egg). This stage happens right after unprotected sexual intercourse when millions of sperm swim to reach the egg. It only takes one sperm to fertilize an egg to form a zygote. Fertilization occurs in the ampulla of the fallopian tube, during which the fusion of the egg’s and sperm’s genetic material takes place. There are 23 chromosomes contained in the nucleus of the ovum and 23 chromosomes in the nucleus of the sperm. Together, these 46 chromosomes undergo changes prior to the mitotic division or mitosis, which leads to the formation of an embryo having two cells.
The fertilized ovum, also known as the zygote, begins to divide into several cells or starts to undergo cleavage. The zona pellucida, which is a strong membrane of glycoproteins, surrounds the two daughter cells. When the zygote undergoes cleavage, it increases the number of cells within the zona pellucida.
During the 8-cell stage of cleavage formation, the embryo undergoes compactation, during which the cells bind tightly to each other to form a compact sphere. After compactation, the embryo moves to the morula stage, which is the 16-cell stage. Following this, cavitation occurs, where the trophoblast, or outermost layer of cells, secrete water into the morula. Cell division continues, increasing the number of cells from 40 to 150. Consequently, a blastocoel (central fluid-filled cavity) forms.
To allow the embryo to increase in volume, the zona pellucida begins a degenerative process, usually happening 4 to 6 days after fertilization. The blastocyst forms at this stage; in the medical field, therefore, this stage is often referred to as the blastula stage or pre-implantation phase of development. This stage lasts approximately until the implantation in the uterus occurs.
The blastocyst is a group of cells known as the inner cell mass, or embryoblast, and the outer cells, the trophoblast. The inner cell mass eventually develops into the amnion, yolk sac and allantois, collectively referred to as the embryo proper. The trophoblast, on the other hand, forms the placenta. The conceptus forms, which is the combination of the embryo and its membranes. Ultimately, the zona pellucida completely disappears to allow the blastocyst to enter the endometrium (wall of the uterus) to implant itself.
The fetal stage begins 8 weeks after fertilization. During this stage, the fetus stays connected to the placenta by the umbilical cord. The umbilical cord acts as a lifeline for the fetus; it supplies the fetus with oxygenated blood that is rich in nutrients coming from the placenta. After deoxygenating and depleting nutrients from the mother’s blood, the umbilical cord returns it to the placenta.
Major anatomical and physiological developments are occurring in the fetal period. During this stage, circulatory, digestive, endocrine, integumentary, lymphatic, muscular, nervous, urinary, reproductive and respiratory systems develop. The fetus transforms in the weeks that follow, which would include changes in the physical appearance by showing a resemblance to the parents, and hair growth appearing on the head, eyebrows and lashes. The baby also uses his motor skills, by sucking his fingers and gripping. The baby also starts to hear sounds and recognizes the voice of his mother. At the end of the 37th week, the baby is at full-term. By the 38th week, the baby is ready to be born.