Conception is the moment when the male gamete, or reproductive cell termed sperm, penetrates the female gamete, the ova. When their nuclei fuse, the new creation is called a zygote. This is the earliest stage of pre-natal development. From this single cell, through complex development emerges that bundle of joy we call a baby.
The first two weeks of development are called the germinal period, also sometimes referred to as the pre-embryonic stage. The third through eighth weeks are the embryonic period and the ninth week through birth is the fetal period. Commonly, the nine-month pregnancy is divided into trimesters. The first trimester lasts through the third month of pregnancy; the second trimester comprises months four, five and six; the third trimester comprises month seven until birth.
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Germinal Period:
Fertilization, or the successful union of sperm and ova, takes place in approximately the third week of pregnancy. The first two weeks are calculated into the pregnancy, dating from the mother’s last menstrual period, or LMP. According to the World Health Organization, a healthy male should deposit at least 40 million sperm with each ejaculation. Out of these, approximately 75% are actually live, viable gametes. Within five minutes sperm can reach the outer edge of the fallopian tube and begin climbing, looking for an ova to penetrate. When this is accomplished, chromosomes from sperm and ova combine 50/50, with each contributing half of the DNA, or genetic information necessary for the zygote to grow and develop into a human infant.
The zygote takes approximately five days to travel down the fallopian tubes in a sort of free-fall, not yet anchored in the uterus. It continues to grow at this time, and cells divide. When the zygote reaches the uterus, it typically has between 60 and fifty cells and is called a morula. At this time, cell differentiation occurs. Outer cells of the zygote create the placenta, umbilical cord and amniotic sac. The inner cells develop into the embryo. The morula develops into a blastocyst that implants into the uterine lining. The placenta begins to form. At this time, hormones signal the mother’s body to shut down the menstrual cycle, and divert energies to sustaining a healthy pregnancy.
All told, an amazingly busy, and productive two-to-three week period.
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Embryonic Period:
Rapid growth continues, as both external and internal organs grow and take shape. The embryo is most at risk during this phase to teratogens. A teratogen is an agent, such as drug and alcohol, or condition, such as stress or malnutrition that can disturb embryonic or fetal development. Some of the embryonic milestones are as follows:
Week 3:
• Brain, spinal cord and heart begin developing.
• Gastrointestinal tract begins developing.
Weeks 4-5:
• Heart begins beating (approximately 3 weeks, one day)
• Brain develops into five areas; some cranial nerves become visible.
• Some bone tissue, including vertebrae, begins to form.
• Spinal cord tissue fuses at mid-point.
• Eyes, ears, nose begin to develop.
• Arm and leg buds form.
• Head tilts forward, and is about 1/3 of the entire embryo.
• Rudimentary blood circulation system in place.
Week 6:
• Arms and legs are longer. Foot and hand areas discernable.
• Fingers and toes present, but webbed.
• Lungs begin developing.
• Embryo begins moving, although the mother may not feel the motion.
Weeks 7-8:
• Nipples and hair follicles form.
• Elbows and toes visible.
• All organs have begun development.
• The heart’s electrical activity similar shows a pattern similar to that of an adult.
• Facial features continue developing.
• Mouth opens and closes.
• Eyes squint.
• Embryo makes hand to face contact.
• Thumb sucking may be observed.
• Swallowing
• May grasp an object.
• Preference for right and left-handedness observable.
• The brain has become highly complex, and makes up approximately 43% of the embryo.
• Testes release testosterone
• Oogonia, basic female ovum, begin development.
• Uterus of the female embryo forms.
This partial list of development illustrates how the growing embryo could be deeply influenced by toxins entering the pre-natal environment. The embryo weighs approximately one thirteenth of an ounce (one gram), and is an inch long.
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Fetal Period:
From the tiny being described at the end of the embryonic period, not much bigger than a thumb joint, the fetus will grow to be an average (single birth) 8-pound, 20-ounce human infant. This kid has a lot of growing still in store before he or she is born. Fetal developmental milestones include:
Weeks 9-12:
• Eyelids close, and remain closed.
• Nose and lips complete.
• Yawning.
• Complex facial expressions observable.
• Limbs are long and thin.
• Genitals are visible and clearly male or female.
• Head makes up ½ the fetuses’ size.
• Tooth buds for baby teeth appear
• Most bones begin to harden.
• Fingernail and toenails begin to grow.
• Unique fingerprints form.
• Taste buds form.
• Palate fuses.
• Bowel movements: Fetus defecates meconium.
• Thyroid gland produces hormones.
• Skin is pink and head begins to grow downy hair called lugano.
• Weight increases nearly 60%.
Weeks 13-16:
• Most of the body becomes touch-sensitive.
• Teeth continue to develop.
• Four-lobed cerebral cortex develops.
• Rooting reflex, necessary for nursing, develops.
• Jaw motions begin; more often in females.
• Fat begins to deposit in face, abdomen, back, kidney, shoulders, arms, and legs.
• Stem cells arrive in bone marrow.
• “Quickening”—mother first feels fetal movement.
Weeks 17-19:
• Fetus can hear.
• Larynx makes speaking movements.
• Insulin is made.
• Number of oogonia peak. All ovum possessed by an adult woman are present.
Week 20:
• Downy lugano hair covers body.
• All skin layers and structures (hair follicles, etc.) complete.
• Fingernails and toenails, growing since the embryonic phase, are now visible.
• Eyelids may begin to separate: eyes open and close.
• Fetus becomes increasingly active.
• Fetal heartbeat can be heard externally with a stethoscope.
Week 21-23:
• Fetus is viable at the end of 22 weeks. With medical intervention, a fetus can survive outside the mother’s womb although the likelihood of severe, life-long health problems increase dramatically if birth occurs this early.
• Bone marrow creates blood cells.
• Lower airways of lungs are developed, but entire lung structure incapable of independent breathing.
• More fat layers throughout the body.
Week 24:
• Fetus has a blink and startle reflex, develops in females before males.
• Lungs continue developing.
Weeks 25-28:
• All parts of the eye completed. Tears are produced.
• Fetus develops a sense of smell.
• Fetus reacts to light.
• Fetus responds to taste.
• Fetus responds to noise outside the womb. Rapid brain growth increases the weight of fetus’s brain 400%-500%.
Weeks 29-32:
• All six layers of cerebral cortex present.
• Rhythmic breathing occurs, but the lungs are still immature.
• Bones fully developed but still soft.
• May show a preference for certain sounds, especially in music.
• Fetus’s body stores iron, calcium, and phosphorous.
• Additional fat deposits begin to fill out fetus’s skin.
Weeks 33-36:
• Prenatal food consumed by mother affects newborn taste.
• Rapid weight gain of infant.
• Lugano begins to disappear, although hair on the head becomes coarse and thicker.
• Fetus develops a firm grip.
Weeks 37-40:
• Amount of amniotic fluid peaks.
• Fetus may drink up to 15 ounces of amniotic fluid daily.
• Fetus drops into lower abdomen and positions head down for birth.
• Fetus initiates labor.
• Birth/ Baby’s transition to air breathing.
Section Bibliography
Berger, K. S. (2005). From Zygote to Newborn. In K. S. Berger, The Developing Person Through the Life Span (6th ed., pp. 91-204). New York, N.Y,, U.S.A.: Worth Publishers.
Pillitteri, A. Maternal & Child Health Nursing, 3rd ed. Philadelphia, PA, USA.
U.S. National Institutes of Health/U.S. National Library of Medicine. (2009, June 17). Medline Plus Fetal Development. Retrieved November 6, 2009, from Medline Plus: http://www.nlm.nih.gov/medlineplus/ency/article/002398.htm
Endowment for Human Development. (n.d.). Endowment for Human Development Interactive Prenatal Development Timeline. Retrieved November 06, 2009, from Endowment for Human Development: http://www.ehd.org/science_main
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