Prenatal Development: Germinal & Embryonic Stages

• Prenatal development includes conception to birth, marked by biological changes
• Prenatal development is commonly divided into three trimesters, each notable for its milestones

Germinal Stage

• The germinal stage is the first and shortest, lasting approximately two weeks, from conception to implantation
• Fertilization sees a sperm penetrate an egg, creating a zygote
• The zygote undergoes cell division via mitosis while traveling down the fallopian tube
• After four days the zygote becomes a blastocyst, a fluid filled sphere of cells
• Around day 10 the blastocyst implants in the uterine wall initiating pregnancy
• Implantation results in hormonal changes to sustain pregnancy and prevent menstruation
• If implantation fails the pregnancy terminates, often without the woman knowing

Embryonic Stage

• The embryonic stage spans from implantation to the eighth week of gestation
• During this time major organs and systems form via organogenesis
• The embryo develops three key layers: the ectoderm (outer), mesoderm (middle), and endoderm (inner)
• The ectoderm develops into skin, hair, sensory organs, nails, and the nervous system
• The mesoderm becomes muscles, skeleton, reproductive organs, blood vessels and the heart
• The endoderm forms the digestive, respiratory system, pancreas, liver and other internal organs
• By the end of the embryonic stage, all major organs and body parts are present, though rudimentary
• The heart begins beating around week five
• Early in this stage the neural tube which becomes the brain and spinal cord develops
• During this period the embryo faces greatest risk from teratogens which can cause birth defects

Fetal Stage

• The fetal stage spans from the ninth week until birth
• This stage features rapid growth and refinement of organ systems
• Size and weight of the fetus increase significantly
• By the end of month three, the fetus can move its limbs and head
• External genitalia are developed, and sex can be determined via ultrasound
• During the second trimester (months 4-6), the mother begins to feel fetal movements (quickening)
• Vernix caseosa, a protective coating, develops on the skin
• Fine hair known as lanugo covers the body
• The brain of the fetus develops quickly
• The organ systems of the fetus continue to mature
• By the end of the second trimester, the fetus has a good chance of survival if born prematurely with intensive medical care
• During the third trimester (months 7-9), the fetus grows rapidly, and organ systems mature
• The lungs mature, preparing to breathe
• The fetus develops antibodies from the mother offering immunity after birth
• The brain continues to develop, and the fetus responds more to external stimuli
• In preparation for birth the fetus generally moves into a head-down position

Teratogens

• Teratogens are environmental agents that can disrupt prenatal development and cause birth defects
• Common teratogens include drugs, alcohol, tobacco, certain medications, environmental toxins, and maternal infections
• A teratogen's effect depends on the dose, exposure timing, and genetic susceptibility of the fetus
• The embryonic stage is the most sensitive to the effects of teratogens, because the major organs are developing
• Alcohol consumption during pregnancy can cause fetal alcohol spectrum disorders (FASDs), which can cause physical, cognitive, and behavioral abnormalities
• Smoking during pregnancy is associated with low birth weight and premature birth as well as an increased risk of sudden infant death syndrome (SIDS)
• Certain medications like thalidomide and isotretinoin (Accutane), are known to cause severe birth defects
• Environmental toxins like lead, mercury, and radiation, can also harm the developing fetus
• Maternal infections, like rubella, cytomegalovirus (CMV), and Zika virus can cause serious birth defects
• Prenatal care and vaccinations, including avoiding teratogens, can lower the risk of birth defects