Human beings develop over many stages, and it begins long before birth. From fertilization to embryonic development and fetal growth, the prenatal period is a busy time for everyone involved. There are three prenatal development stages between conception and delivery: the germinal stage, the embryonic stage, and the fetal stage.
Let's review all three stages of prenatal development, looking at key processes, important milestones, and lifestyle considerations.
Germinal Stage
The first stage of prenatal development is the germinal stage. It's the shortest stage of pregnancy, and it's also the most sensitive. The life process starts at conception inside the fallopian tube, when the sperm joins and fertilizes the egg. Now called a "zygote," it travels down the uterus and divides many times over 8-9 days until it forms an embryo.
The following micro-processes occur during this period:
Fertilization
During fertilization, a multitude of sperm travel towards the egg. One sperm succeeds in fertilizing the ovum, penetrating its cell membrane and depositing its genetic material. The fertilized ovum is known as the zygote, and it remains in the fallopian tube for 72 hours.
Cleavage
By the second day after fertilization, the single-celled zygote divides to form two daughter cells. This process is known as mitosis, and it continues over 12-24 hours. By day four there are typically 16 cells, which form a solid ball called a morula. This is the same size as the initial zygote, but it's much more complex.
Blastulation
This process sees the solid morula change into a hollow ball of differentiated cells. This fluid-filled structure is called the "blastocyst," and it includes the embryoblast, trophoblast, and blastocoel. The trophoblast is the outer cell layer, with its cells implanting in the uterus to become the placenta.
Implantation
During the final stage, the blastocyst becomes embedded in the endometrium of the uterus. It's triggered by contact with the endometrium, secreting enzymes that enable penetration. Projections pull the blastocyst into the endometrium, with the final cell mass now known as the embryo.
Embryonic Stage
Now starts the embryonic stage of pregnancy, which lasts from the second week to the eighth week after fertilization. A number of milestones are reached during this period, from the onset of the first heartbeat to the development of fingers, toes, neural structures, and major organs.
The following micro-processes are the most important:
- Early development: This period starts in the second week. Distinct nervous system, blood, and organ cells are developed over many days, with the embryo growing in size and complexity as it undergoes mitosis.
- Gastrulation: From the latter half of the second week, the blastula layer folds inward to create a gastrula. The inner layer or endoderm forms the internal glands and organs; the middle layer or mesoderm forms the heart, bones, and muscles; and the outer layer or ectodern forms the nervous system, eyes, and ears.
- Neurulation: Taking place in weeks three and four, this process is largely responsible for the creation of the nervous system. Neurolation sees the formation and development of the ectoderm, which folds inwards to form a neural tube. This becomes the spinal cord, with one end bulging to become the brain.
- Organogenesis: During weeks three to eight, several complex organs develop. Primitive blood vessels develop into the heart, which starts to pump blood and forms five distinct regions.
- Later development: The face and arms start to take shape in week five, the eyes and legs can be seen in week six, and the sex organs and lungs develop in week seven. By week eight, all essential organs have begun to form and a heartbeat can be detected.
Fetal Stage
The prenatal development stages continue with the fetal stage, which extends from the ninth week to the 38th. The embryo undergoes an amazing transformation during this period, growing in size from just over an inch to more than 20 inches.
The fetal stage is divided into the following periods:
Weeks 9-15
During the early weeks of this stage, the fetus develops quickly and becomes more active. The sex organs are differentiated by week 12, and the biological sex of the fetus can be determined accurately. Facial features also continue to develop, and fine hair called lanugo grows on the fetus's face. Internally, the thyroid gland and kidneys start functioning, with the fetus swallowing amniotic fluid and passing urine.
Weeks 16-26
The middle period of the fetal stage involves several important changes. The brain and nervous system develop to create a sense of touch, the eyes and ears move to their final position, and the bones begin to ossify and produce blood cells. The lungs and intestines also develop, but the fetus is still dependent on the mother for nutrition. Significant muscle development also takes place, with movements becoming more refined and powerful.
Weeks 27-38
The final stage of development sees the fetus grow in size and complexity. Some changes help the fetus to survive in the outside world, with body fat increasing dramatically and head hair growing thicker and stronger. Breathing is also supported, with the fetus learning to move the diaphragm in anticipation of its first breath. Other changes aid perception, with the fetus now able to feel physical sensations, hear sounds, and detect light.
Genetic Influences
Genetics play a significant role during all stages of prenatal development. A number of genetic problems are known to affect healthy growth, including Down syndrome or trisomy 21, inherited diseases like sickle-cell anemia and cystic fibrosis, and sex-chromosome problems such as Klinefelter's syndrome and Turner syndrome. Genetic tests are carried out to identify these issues, with various screening and diagnostic tests offered during the first and second trimester.
Environmental Influences
Environmental factors play a key role in prenatal development. The mother's health has a massive impact, from pre-existing health conditions to nutrition and lifestyle decisions made during pregnancy. Smoking during pregnancy can cause a wide range of serious problems, including miscarriage and premature labor. Alcohol consumption is also harmful, with exposure linked with fetal alcohol spectrum disorder and other conditions. Infection by rubella (German measles) can also have serious consequences, as can exposure to radiation from diagnostic X-rays and therapies.
Frequently Asked Questions
What are the 3 stages of prenatal development?
Prenatal development is often divided into three phases: the germinal stage, the embryonic stage, and the fetal stage.
What happens during the germinal stage?
The germinal stage involves cell division or mitosis, which ends when the embryo is created in the uterus.
What are the key milestones reached in the embryonic stage?
A number of milestones are reached during this stage, from the development of the heart to important physical and neural structures.
What differentiates the fetal stage from previous stages?
The fetal stage is the longest of the three, and it's characterized by the growth and transformation of existing biological structures.
What should expectant mothers do to support healthy prenatal development?
The best way to support prenatal development is to follow a healthy lifestyle, avoid harmful exposure, and follow recognized health guidelines.
Final Thoughts
Prenatal development takes place over 38 weeks, as new life transforms and grows inside the womb. This complex process is divided into three stages, with each one longer than the last. The germinal stage sees new cells divide and implant themselves inside the uterus. The embryonic stage sees the development of major organs and physical structures. And the fetal stage sees further differentiation and growth in preparation for birth.
While so much goes on behind the scenes during pregnancy, proactive moves are possible to support healthy prenatal development. To support the health of your baby and yourself during this critical time, it's important to make good decisions and maintain regular check-ups based on health guidelines.
