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Understanding Twins: Formation and Classification
The term “twins” means the delivery of more than one offspring during a single birth. Multiple births are fairly common in mammals, but specifically with humans, it is not as common (Britannica 2025). “Twin birth accounts for 2 to 4% of total births” (Santana et al., 2018). Twins are categorized into two specific types: Monozygotic (identical) and Dizygotic (fraternal). The differences between the two types of twins lie in the formation of the zygote. In Monozygotic twins, the fertilized ova merely split into two; this egg is fertilized by the same sperm, resulting in two identical offspring. On the other hand, in Dizygotic twins, the ovaries produce two ova, which are each fertilized by separate sperm, resulting in different DNA, forming fraternal twins (Osmosis from Elsevier 2023).
Twins alter the fetal development process, resulting in the production of multiple offspring at once. Throughout pregnancy, the offspring, known as fetuses, grow from small zygotes to infants. In the case of twins, either two separate eggs are fertilized or a single fertilized egg is split into two embryos (Tarnoki and Tarnoki). This results in the development of two fetuses simultaneously. The first trimester is the first 12 weeks of pregnancy. In this trimester, the fertilized egg, or the zygote, changes from a small group of cells to a fetus with noticeable human features. Throughout the first four weeks, your uterus prepares for a potential pregnancy during the follicular phase of the menstrual cycle (Cleveland Clinic 2024). This is when the follicle-stimulating hormone (FSH), estrogen, progesterone, and luteinizing hormone (LH) are released to support the potential pregnancy (Magon and Kumar, 2012). After the ova and sperm come together to create a zygote, which turns into a blastocyst with a water-tight sac around it, implanting into the uterine lining, helping to form the placenta and the amniotic sac. In pregnancies with two fetuses, two blastocysts either implant separately or share a placenta. This implantation initiates embryonic development. Near the end of the first trimester, the embryos form the most essential structures, such as the heart, limbs, and the neural tube. At the very end of the first trimester, the embryos are now fetuses containing all major organs. The first trimester is the stage where the body undergoes several hormonal and cellular changes to prepare for the physical changes required in the future (Cleveland Clinic, 2024).
The second trimester includes weeks 13 through 28. During this time, the fetuses develop more distinct human-like features. These features include fully formed limbs and facial definition. Along with the physical changes, the fetuses now develop more sensory abilities, such as auditory and tactile responses. Although all organs are fully developed at this time, the circulatory and respiratory systems are still fully developing in fetuses (Cleveland Clinic 2024). In twin pregnancies, these developments occur in both fetuses together, which may result in more prominent changes in the carrier’s body. This is due to more nutritional demands along with the lack of space. Movement within the placenta from the baby also occurs at this time, which may be more noticeable with twins. Throughout the second trimester, the fetuses go through many anatomical and physical changes.
The last stage of pregnancy, or the third trimester, is when the fetuses are in the final preparation stages for life outside the placenta. During this time, the fetuses see a greater amount of neurological development, specifically in the cerebral cortex. This is the area of the brain responsible for enhancing the ability for motor control and the processing of sensory information. The fetuses, now the size of infants, rotate within the placenta to a “head-down” position to prepare for labor. By week 37, the fetus has all anatomical characteristics fully grown, and by 40 weeks, but twins are commonly conceived before this milestone due to space constraints (Cleveland Clinic 2024).
Twins are rare but intriguing occurrences, altering the fetal development process in several ways. The fetal development process occurs in the same but for two offspring at the same time. In the abstract, the fetal development process was illustrated, displaying each month of growth. This correlates with the description of the major fetal development steps displayed throughout this essay, following the growth from month one until month nine. With twins specifically, the fetal development process, pregnancy length, and embryo formation are altered to accommodate the needs of the fetuses throughout their gestation period.
Works Cited
Britannica. (2025). Multiple Birth | Description, Causes, & Occurrences. Encyclopedia Britannica. https://www.britannica.com/science/multiple-birth.
Cleveland Clinic. (2024, March 19). Fetal Development. Cleveland Clinic. https://my.clevelandclinic.org/health/articles/7247-fetal-development-stages-of-growth
Magon, N., & Kumar, P. (2012). Hormones in Pregnancy. Nigerian Medical Journal, 53(4), 179.
https://pmc.ncbi.nlm.nih.gov/articles/PMC3640235/
Osmosis from Elsevier. (2023). Development of twins. YouTube. https://www.youtube.com/watch?v=ERMt3H6f9BI
Santana, D., Surita, F., & Cecatti, J. (2018). Multiple Pregnancy: Epidemiology and Association with Maternal and Perinatal Morbidity. Revista Brasileira de Ginecologia E Obstetrícia / RBGO Gynecology and Obstetrics, 40(09), 554–562. https://pmc.ncbi.nlm.nih.gov/articles/PMC10316907/.
Tarnoki, D. L., & Tarnoki, A. D. (2022). Twin Research and Imaging. Medicina, 58(6), 805. https://pmc.ncbi.nlm.nih.gov/articles/PMC9228945/