What is Prenatal Development

Prenatal development is the series of changes an organism goes through in the womb as it progresses from single-celled zygote to fully formed baby. During this time, cells divide continually and create more and more tissue. The tissue differentiates into specialized organs and structures. This process is controlled and regulated by the fetus’s DNA and the hormones and signals it produces. Prenatal development in humans is broken into three separate trimesters, or stages, through which the fetus must develop. Those stages are discussed below.

Prenatal Development Stages

Human prenatal development is broken into three trimesters, as seen in the graph below.

Stages in Prenatal development

First Trimester

Pregnancies are measured from a woman’s last menstruation. Menstruation signals the release of the old uterine lining, and the beginning of a new reproductive cycle. This cycle starts with the release of an egg, or oocyte, from the ovaries into the fallopian tube. In order to get pregnant, a woman must have sex 5 days before this event, as that is as long as sperm can survive in the female reproductive tract. Also, the egg can only survive for 2-3 days once outside of the ovary. Therefore, there is a very narrow window in which fertilization can occur.

Because prenatal development is tracked from the last menstruation, fertilization occurs at 1-2 weeks. This event usually happens in the fallopian tube. The sperm meets the eggs, and the two fuse together to form the zygote. As the zygote makes its way to the uterus, it begins to develop and divide. After around a week of slowly traveling down the fallopian tube, the single cell has become a hollow ball of cell, called the gastrula. This ball needs to implant onto the wall of the uterus if it is to survive. These initial stages, known collectively as embryogenesis, carry the new organism from a single cell all the way to a fetus. The process is shown in the image below.

embryogenesis in humans

During this trimester of prenatal development, the embryo is especially sensitive to any environmental toxins or chemicals that the mother comes in contact with. This is especially true after implantation, when the small embryo begins to receive nutrients from the mother.

By weeks 5 and 6, the embryo is starting to develop the advanced organ systems which the fetus requires. A primitive heart starts circulating fluids around the embryo. Other organ systems, like the nervous system and digestive system, also form as the cells continue to divide and fold into special shapes.

By weeks 11 to 12, the embryo is becoming a fetus, with most of its organ systems intact and mostly functioning. Many changes must still happen before birth however. While the fetus is almost fully formed, it is still tiny. At this point in prenatal development, the fetus is only around 3 inches long. It will be a full 19 to 20 inches before it is born.

Second Trimester

During the second trimester of prenatal development, the fetus begins to finish constructing the organ systems. During this time all of the organ systems advance. The liver, pancreas, spleen, and other secretory organs begin producing fluids. Red blood cells begin to be produced, and the muscles and bones strengthen. As these organ systems form, the fetus becomes less susceptible to damage from toxins and carcinogens. This is because the majority of the cells in the body have already differentiated, and the precursors to the organs are already formed. Therefore, the toxin may do minor damage to one of these systems, but it will not disrupt the entire prenatal development as easily as it can during the embryo stage.

By the end of this trimester, the baby is considered almost fully developed. In fact, babies delivered at only week 24 still have a 50% chance of surviving if given the proper treatment for a premature baby. This treatment includes confining the newborn to an intensive care unit to be carefully monitored as they finish developing. At this point, the fetus has gained control of its body through the development of the nervous system. By week 26, the fetus is entering the third trimester, which is all about gaining weight and preparing to enter the world.

Third Trimester

The third trimester of prenatal development is almost all about growth. The fetus begins to store up large amounts of fat all over the body. Some of this fat is specialized brown adipose tissue, which will help the baby stay warm after it is born. Other fat deposits will be used for energy to continue growing. The brain and neurons continue to develop during this time, and the baby can even use their senses of touch and hearing to begin to understand the outside world.

Babies born within this trimester have an ever-increasing chance of survival as they near “full term” or 36 weeks. Any time after that, and the baby should survive. From the beginning of the third trimester all the way up to 36 weeks, the baby has a chance at surviving if born early. Along with fat, newborns tend to accumulate hair during this stage of prenatal development, which will also serve to keep them warm.

The end of prenatal development comes with birth. On average for humans, this comes at around 40 to 41 weeks after the woman’s last menstruation. At the end of week 42, the baby is considered post mature. When this happens, doctors may choose to induce labor or surgically remove the baby to protect the mother and the baby.

Quiz

1. Which of the following is NOT a stage of prenatal development?
A. First Trimester
B. Embryogenesis
C. Gametogenesis

Answer to Question #1
C is correct. Gametogenesis happens when an adult body creates gametes for reproduction. This is separate from prenatal development, but the gametes created by this process start the process of prenatal development.

2. Which of the following organisms does not undergo prenatal development?
A. Whale
B. Fish
C. A single celled yeast

Answer to Question #2
C is correct. By definition, prenatal development can only happen in multicellular species which are created with sexual reproduction. While the process may differ significantly from that of humans, there is always an egg, and the embryo always develops inside of it. The egg may or may not be housed in the uterus, and may be laid in a nest to develop. While the method of getting an egg to survive may change, the actual prenatal development of these organism is fairly similar.

3. An elephant has a slightly different timeline for prenatal development. Instead of 9 months, they carry their fetuses for nearly 22 months, or almost 2 years. Why is this?
A. Elephant babies are much larger than humans
B. The elephant brain must be more developed
C. Elephants have slower prenatal development

Answer to Question #3
B is correct. When you think about the size of a baby elephant compared to its mother, that size difference is comparable to the size difference between a human mother and her baby. So, although the elephant baby is several hundred pounds heavier than the human baby, it is still relatively tiny compared to its mother. However, compared to human babies, the elephant baby’s brain is much more developed. Over time the human will far surpass the elephant, but at birth elephants need to be able to walk, call, locate food, and communicate with their social group. The extra time in the womb keeps them safe and protected as they develop their brain.

References

  • Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammology: Adaptation, Diversity, Ecology(3rd ed.). Baltimore: The Johns Hopkins University Press.
  • Pough, F. H., Janis, C. M., & Heiser, J. B. (2009). Vertebrate Life. Boston: Pearson Benjamin Cummings.