The progressive growth that occurs between fertilization of an egg to the birth of a baby.
Pre-embryonic stage
The pre-embryonic stage starts with fertilization and lasts through the first two weeks of pregnancy.
FERTILIZATION. During intercourse, the male ejaculates and releases semen into the woman's vagina. The semen contains 50 to 200 million spermatozoa per milliliter that reach the cervix within 90 seconds and the outer end of the fallopian tubes within five minutes. Fertilization results when a single sperm, or spermatozoon, penetrates an ovum. The chromosomal material of the ovum and spermatozoon then combine, forming a zygote.
IMPLANTATION. After fertilization, the zygote travels three or four days through the fallopian tube toward the body of the uterus. During this time, the cell starts to divide. By the time the zygote reaches the body of the uterus, it consists of 16 to 50 cells and is called a morula. The morula collects large cells at the periphery of the ball and becomes an outer casing with a connected inner group of cells surrounded by a fluid space. At this stage, the structure is termed a blastocyst. The blastocyst implants on the inner layer of the uterus, called the endometrium, approximately eight to 10 days after fertilization, where it will obtain nourishment. In as many as 50% of all pregnancies, the zygote fails to reach the implantation stage, in which it becomes an embryo.
Embryonic stage
The embryonic stage spans the third week to the eighth week of pregnancy.
Body organs form out of layers of tissue called germ cells. The three distinct layers of germ cells are called the endoderm, the ectoderm, and the mesoderm. Newborns with a congenital defect originating from one of the three layers should be evaluated for malformations that developed
from the same germ cells. For example, the heart and kidneys form from the mesoderm. A baby born with a heart defect should also have an x ray or ultrasound of the kidneys.
During the embryonic period, the baby is most susceptible to invasion by teratogens, which are substances that can result in birth defects. Women should be instructed on ways to reduce their contact with teratogens.
Milestones of fetal development
The life of the fetus is commonly calculated from the time of ovulation or fertilization (ovulation age), but the duration of the pregnancy is usually calculated from the first day of the last menstrual period (gestational age). The following outline of fetal development milestones is based on 40 weeks of gestation.
END OF FOUR WEEKS GESTATION.
Fetus reaches a length of 0.75 to 1 cm and weighs 400 mg.
Spinal cord forms and fuses at the center.
Lateral wings bend forward meeting at the center and will eventually form the body.
Head tilts forward and makes up about one-third of the entire structure.
The rudimentary heart beats a regular rhythm.
Arms and legs have the appearance of small buds.
The beginnings of eyes, ears, and a nose are evident.
END OF EIGHT WEEKS GESTATION.
Fetus reaches a length of 1 in (2.5 cm) and weighs 20 g.
The heart has a definite septum and valves.
Extremities have lengthened.
External genitalia are evident, but gender is not obvious.
END OF 12 WEEKS GESTATION.
Fetus reaches a length of 2.8-3.6 in (7-9 cm) and weighs 45 g.
Some movement occurring, but usually too faint for the mother to feel.
Fetal heart can be heard with an electronic device called a Doppler.
END OF 16 WEEKS GESTATION.
Fetus reaches a length of 4-7 in (10 to 17 cm) and weighs 55 to 120 g.
Liver and pancreatic secretions are present.
Fetus starts to make sucking motions with the mouth.
END OF 20 WEEKS GESTATION.
Fetus reaches a length of 10 in (25 cm) and weighs 223 g.
The mother starts to feel fetal movement.
Fetal heart tones can be heard with a stethoscope.
END OF 24 WEEKS GESTATION.
Fetus reaches a length of 11-14 in (28-36 cm) and weighs 550 g.
Eyebrows and eyelashes are clearly formed.
Eyelids, which fused in the 12th week, start to open.
Surfactant, a phospholipid substance essential to lung function, is formed and excreted by cells in the alveoli.
END OF 28 WEEKS GESTATION.
Fetus reaches a length of 14-15 in (35-38 cm) and weighs 1200 g.
Testes begin descent into the scrotal sac from the lower abdominal cavity if the fetus is male.
The brain is rapidly developing.
END OF 32 WEEKS GESTATION.
Fetus reaches a length of 15-27 in (38-43 cm) and weighs 1600 g.
Fetus hears sounds and responds with movement.
Delivery presentation (vertex or breech) may be assumed.
Iron stores begin to develop.
END OF 36 WEEKS GESTATION.
Fetus reaches a length of 17-20 in (42-49 cm) and weighs 5-6 lbs (1900-2700 g).
Soles of the feet have only one or two creases.
The central nervous system has greater control over body functions.
END OF 40 WEEKS GESTATION.
Fetus reaches a length of 19-21 in (48-52 cm) and weighs 7-7.5 lbs (3000 g).
Fingernails have grown over the fingertips.
There are creases covering at least two-thirds of the soles of the feet.
Fetus kicks vigorously and may cause the mother discomfort.
Role in human health
Knowledge of fetal development can assist the mother and other family members in visualizing the fetus at the various stages of development. The parent-infant bonding that results can be an incentive for the mother to engage in healthful behaviors.
KEY TERMS
Fetus—Infant during intrauterine life.
Sonogram—Ultrasound; uses sound waves to take pictures and view the fetus.
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