2.7: Periods of Prenatal Development

Prenatal development lasts for 9 months or 40 weeks. From the point of view of laypersons and the parents, we often divide those into three month periods or trimesters. However, from the scientific viewpoint of prenatal development, this is divided into three unequal parts - the first 2 weeks, 2 weeks to 2 months, and the last 7 months.

The Germinal Period

The germinal period (about 14 days in length) lasts from conception to implantation of the fertilized egg in the lining of the uterus. At ejaculation millions of sperm are released into the vagina, but only a few reach the egg and typically only one fertilizes the egg. Once a single sperm has entered the wall of the egg, the wall becomes hard and prevents other sperm from entering. After the sperm has entered the egg, the tail of the sperm breaks off and the head of the sperm, containing the genetic information from the biological male, unites with the nucleus of the egg. As a result of fertilization, a new cell is formed. This cell, containing the combined genetic information from both biological parents, is referred to as a zygote. It is typically fertilized in the top section of the fallopian tube and continues its journey to the uterus.

Figure \(\PageIndex{1}\): The ovum is one of the largest human cells and at conception it is fertilized by one of many released sperm.[1]

As it travels down the fallopian tube, the organism begins cell division through mitosis (the process of regular growth where a cell makes a copy of itself). After five days of mitosis there are 100 cells, which is now also called a blastocyst. The blastocyst consists of both an inner and outer group of cells. The inner group of cells, or embryonic disk will become the embryo, while the outer group of cells, or trophoblast, becomes the support system which nourishes the developing organism. This stage ends when the blastocyst fully implants into the uterine wall (U.S. National Library of Medicine, 2015).

Figure \(\PageIndex{2}\): The cycle of fertilization – on day 0 at the point of ovulation an oocyte (egg) is released from the ovary. When sperm are present in the fallopian tubes, one might fertilize the ovum which is then called a zygote. On day 1 the first cleavage occurs, and by day 3-4, there are 4 cells which turn into an 8 cell uncompacted morula, and an early blastocyst by day 5. All this happens while the zygote is traveling down the fallopian tubes into the uterus. By day 6-7 there is “hatching” and by day 8-9, the blastocyst “implants” into the uterine wall.[2]

Mitosis is a fragile process and fewer than one half of all zygotes survive beyond the first two weeks (Hall, 2004). Some of the reasons for this include: the egg and sperm do not join properly (thus their genetic material does not combine), there is too little or damaged genetic material, the zygote does not replicate, or the blastocyst does not implant into the uterine wall. The failure rate is higher for in vitro conceptions. The figure illustrates the journey of the ova from its release to its fertilization, cell duplication, and implantation into the uterine lining.[3]

The Embryonic Period

Starting in the third week, the blastocyst has implanted in the uterine wall. Upon implantation this multi-cellular organism is called an embryo. Now blood vessels grow forming the placenta. The placenta is a structure connected to the uterus that provides nourishment and oxygen from the pregnant individual to the developing embryo via the umbilical cord.

During this period, cells continue to differentiate. Growth during prenatal development occurs in two major directions: from head to tail called cephalocaudal development and from the midline outward referred to as proximodistal development. Cephalocaudal means that those structures nearest the head develop before those nearest the feet (actually “caudal” refers to tail). Proximodistal means that those structures nearest the torso develop before those away from the center of the body (such as hands and fingers). You will see that these patterns continue after birth.

The head develops in the fourth week and the precursor to the heart begins to pulse. In the early stages of the embryonic period, gills and a tail are apparent. However, by the end of this embryonic stage they disappear and the organism takes on a more human appearance.

Figure \(\PageIndex{3}\): A human embryo has a disproportionately large head, very small beginning of limbs, and looks less like a human than it will begin to later.[4]

About 20 percent of organisms fail during the embryonic period, usually due to gross chromosomal abnormalities, often before the pregnant person even knows that they are pregnant. It is during this stage that the major structures of the body are taking form, making the embryonic period the time when the organism is most vulnerable to the greatest amount of damage if exposed to harmful substances. Prospective parents are not often aware of the risks they introduce to the developing embryo during this time. The embryo is approximately 1 inch in length and weighs about 4 grams at the end of eight weeks. The embryo can move and respond to touch at this time.[5]

The Fetal Period

From the ninth week until birth (which is forty weeks for a full-term pregnancy), the organism is referred to as a fetus. During this stage, the major structures are continuing to develop.

Figure \(\PageIndex{4}\): Fetus in amniotic sac that looks very similar to an embryo at first flush

When the organism is about nine weeks old, the embryo is called a fetus. At this stage, the fetus is about the size of a kidney bean and begins to take on more human-like form as the “tail” begins to disappear.

From 9–12 weeks, the sex organs begin to differentiate. By the 12th week, the fetus has all its body parts including external genitalia. In the following weeks, the fetus will develop hair, nails, teeth, and the excretory and digestive systems will continue to develop. At the end of the 12th week, the fetus is about 3 inches long and weighs about 28 grams.

At about 16 weeks, the fetus is approximately 4.5 inches long. Fingers and toes are fully developed, and fingerprints are visible. During the 4-6th months, the eyes become more sensitive to light, and hearing develops. The respiratory system continues to develop. Reflexes such as sucking, swallowing, and hiccupping develop during the 5th month. Cycles of sleep and wakefulness are present at that time as well. Throughout the fetal stage, the brain continues to grow and develop, nearly doubling in size from weeks 16 to 28. The majority of the neurons in the brain have developed by 24 weeks although they are still rudimentary and the glial or nurse cells that support neurons continue to grow. At 24 weeks the fetus can feel pain. [6]

The first chance of survival outside the womb, known as the age of viability is reached at about 24 weeks.[7] By the time the fetus reaches the sixth month of development (at about 24 weeks), it weighs up to 1.4 pounds. The hearing has developed, so the fetus can respond to sounds. The internal organs, such as the lungs, heart, stomach, and intestines, have formed enough that a fetus born prematurely at this point has a chance to survive outside of the womb.

Between the 7th and 9th months, the fetus is primarily preparing for birth. It is exercising its muscles, its lungs begin to expand and contract. It is developing fat layers under the skin. The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy which includes a layer of fat gained during the 8th month. This layer of fat serves as insulation and helps the baby regulate body temperature after birth.

Around 36 weeks, the fetus is almost ready for birth. It weighs about 6 pounds and is about 18.5 inches long, and by week 37 all of the fetus’s organ systems are developed enough that it could survive outside the uterus without many of the risks associated with premature birth. The fetus continues to gain weight and grow in length until approximately 40 weeks. By then, the fetus has very little room to move around and birth becomes imminent.

Figure \(\PageIndex{5}\): Figure – A human fetus with fingers, toes differentiated, ribs visible, as also the amniotic sac and umbilical cord.[8]

Figure \(\PageIndex{6}\): The development of a fetus showing all the milestones at 9 then 12 weeks and every 4 weeks after that.[10]

Attributions:

Child Growth and Development by Jennifer Paris, Antoinette Ricardo, and Dawn Rymond, 2019, is licensed under CC BY 4.0

Parenting and Family Diversity Issues by Diana Lang, 2020, published by Iowa State University is licensed under CC BY-NC-SA 4.0

[1] Image is in the public domain

[2] Image by Ttrue12 is licensed under CC BY-SA 3.0

[3] Lifespan Development: A Psychological Perspective (pages 42-43) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[4] Image by Anatomist90 is licensed under CC BY-SA 3.0

[5] Lifespan Development: A Psychological Perspective (page 43) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[6] Royal College of Obstetricians and Gynecologists. (1997). Fetal Awareness: Report of a Working Party. London: RCOG Press.

[7] Morgan, M.A., Goldenberg, R.L., & Schulkin, J. (2008) Obstetrician-gynecologists' practices regarding preterm birth at the limit of viability. The Journal of Maternal-Fetal & Neonatal Medicine, 21(2), 115-121. doi: 10.1080/14767050701866971

.[8] Image by lunar caustic is licensed under CC BY-SA 2.0

[9] Lifespan Development: A Psychological Perspective (pages 44-45) by Martha Lally and Suzanne Valentine-French is licensed under CC BY-NC-SA 3.0

[10] Image by CNX Psychology is licensed under CC BY 4.0