Prenatal Development
Pregnancy is primarily divided into three-month stages called trimesters. The first trimester (fertilization to week 12) is mainly characterized by cleavage, implantation, embryogenesis, and organogenesis. The fertilized egg, zygote, undergoes cell division and develops into a blastocyst that attaches itself to the uterine wall in a process called implantation. The cells arrange themselves into three germ layers (embryogenesis) and then differentiate into organs and organ systems (organogenesis): Prenatal Development.
The limbs, nose, ears, and eyes are formed, the heart begins to beat, and sexes can be differentiated. By week 12, the fetus is about 9 cm and 45 g. For the mother, metabolic demands increase owing to hormonal shifts. For instance, the secretion of the human chorionic gonadotropin (HCG) peaks between weeks 10 and 12, and the amount of estrogen and progesterone increases (Saladin, 2021). The mother also experiences nausea and fatigue.
In the second trimester (week 13 to week 24), organs complete their development significantly. By week 16, sucking movements are observable, and heartbeats can be heard with a stethoscope. By week 20, the fetus is covered by lanugo and vernix caseosa for protection from the amniotic fluid. The fetus moves into the fetal position, and the mother can experience fetal movements.
At week 24, the eyes open partially and the skin appears pink, translucent, and wrinkled. The fetus is about 23 cm and 820 g (Saladin, 2021). Major events for the mother include reduced nausea, increased energy, and weight gain. Stretching of the dermis also results in striae and linea nigra.
Lastly, the third trimester (week 25 to birth) is marked by rapid growth of the fetus and adequate cellular differentiation of organs. Lung function improves, eyes are fully opened, hair is present on the head, and the fetus moves to an upside-down vertex position. Subcutaneous fat deposition occurs, lanugo is shed, and the skin is less wrinkled (Saladin, 2021). The fetus obtains nutrients from the placenta and their weight is approximately 3,400 g by week 38.
For the mother, the cardiac output increases by approximately 30% to 40% above the normal ranges, the pelvis expands adequately so that the fetus can move lower in the abdominopelvic cavity, and the uterus weighs around 900 g. Strong Braxton Hicks contractions are experienced, and by the end of this trimester, powerful labor contractions occur, leading to childbirth.
The three germ layers that form via gastrulation give rise to the major embryonic structures and extra-embryonic fluids. Gastrulation is the reorganization of an embryo into an organism primarily containing three germ layers—endoderm (innermost), mesoderm (middle), and ectoderm (outermost) (Muhr et al., 2023). Endoderm forms the mucosae of digestive and respiratory tracts and their associated glands (except salivary glands), urinary bladder, and parts of the urethra.
Mesoderm forms bones and bone marrow, muscle tissue, adrenal cortex, dermis, middle ear, blood, lymphatic vessels and tissue, epithelia of ureters, kidneys, gonads and genital ducts, and mesothelium of thoracic and abdominal cavities (Saladin, 2021). Lastly, the ectoderm forms the epidermis and nervous system.
References
Muhr, J., Arbor, T. C., & Ackerman, K. M. (2023, April 23). Embryology, gastrulation. StatPearls – NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK554394/
Saladin, K. (2021). Anatomy and physiology: The unity of form and function (9th ed.). McGraw-Hill Education.
ORDER A PLAGIARISM-FREE PAPER HERE
We’ll write everything from scratch
Question 
Initial Post Instructions
- Describe the major events that occur during the development of offspring and to a birthing parent during each of the three major prenatal periods (trimesters). Then, explain how the three general germ layers that form on day 15 will form the major structures of not only the developing offspring, but the extra-embryonic membranes.
Prenatal Development