STAGES OF FETAL DEVELOPMENT

Pre-embryonic Stage (Conception to Week 2)

Embryonic Stage (Week 3 to Week 8

Fetal Stage (Week 9 to Birth)

Prenatal Development Periods

Pre-embryonic Stage (Conception to Week 2)

: Fertilization occurs in Ampulla → Zygote undergoes cleavage to form Morula (16 cells) → Enters uterus day 3–4 → Becomes Blastocyst (inner cell mass = embryo; outer cell mass = trophoblast/placenta) → Implantation into endometrium (now called Decidua) on days 6–10

Embryonic Stage (Week 3 to Week 8)

: CRITICAL PERIOD OF ORGANOGENESIS. Highly susceptible to teratogens. All major organ systems are established by end of week 8.

Fetal Stage (Week 9 to Birth)

: Period of growth, maturation, and refinement of established organ systems.

ECTODERM

MESODERM

ENDODERM

Germ Layer

ECTODERM

Nervous system, epidermis, hair, nails, tooth enamel, sensory epithelium of eyes/ears/nose.

MESODERM

Muscles, bones, cartilage, connective tissue, cardiovascular system, blood cells, kidneys, gonads.

ENDODERM

GI tract lining, respiratory tract lining, liver, pancreas, thyroid, bladder epithelium.

Week 4

Heart begins to beat and pump blood; neural tube closes; arm/leg buds appear.

Week 8

All organ systems formed; facial features developing; human appearance; heart beats steadily

Week 12

Fetal heart tones heard via Doppler; sex recognizable by genitalia; urine formation begins.

Week 16–20

Quickening felt by mother (multipara ~16w, primipara ~20w); Vernix caseosa & Lanugo present

Week 24

Limit of viability; Alveoli forming; Surfactant production begins; Primitive respiratory movements

Week 28

Eyes open/close; surfactant increasing; baby can survive if born with intensive care

Week 32–36

Subcutaneous fat deposits; L/S ratio reaches 2:1 by week 35–36 (indicates lung maturity).

Fertilization

(also referred to as conception and impregnation) is the union of an ovum and a spermatozoon. This usually occurs in the outer third of a fallopian tube, the ampullar portion (Crombleholme, 2009).

72 hours

the total critical time span during which sexual relations must occur for fertilization to be successful is about _______ (48 hours before ovulation plus 24 hours afterward)

Ovum

From ovulation to fertilization

Zygote

From fertilization to implantation

Embryo

From implantation to 5–8 weeks

Fetus

From 5–8 weeks until term

Conceptus

Developing embryo or fetus and placental structures throughout pregnancy

Age of viability

The earliest age at which fetuses could survive if they were born at that time, generally accepted as 24 weeks, or fetuses weighing more than 400 g

Implantation

or contact between the growing structure and the uterine endometrium, occurs approximately 8 to 10 days after fertilization. After the third or fourth day of free floating (about 8 days since ovulation), the blastocyst sheds the last residues of the corona and zona pellucida. The structure brushes against the rich uterine endometrium (in the second [secretory] phase of the menstrual cycle), a process termed apposition. It attaches to the surface of the endometrium (adhesion) and settles down into its soft folds (invasion).

Decidua basalis

Decidua capsularis

Decidua vera

The decidua has three separate areas

Decidua basalis

, the part of the endometrium that lies directly under the embryo (or the portion where the trophoblast cells establish communication with maternal blood vessels)

Decidua capsularis,

the portion of the endometrium that stretches or encapsulates the surface of the trophoblast

Decidua vera,

the remaining portion of the uterine lining

chorionic villi

As early as the 11th or 12th day, miniature villi that resemble probing fingers, termed _________, reach out from the single layer of cells into the uterine endometrium to begin formation of the placenta. At term, almost 200 such villi will have formed

syncytial layer

A double layer of trophoblast cells surrounds these. The outer of the two covering layers is the syncytiotrophoblast, or the _________. This layer of cells produces various placental hormones, such as hCG, somatomammotropin (human placental lactogen [hPL]), estrogen, and progesterone.

Langhans’ layer

The middle layer, the cytotrophoblast or ________, is present as early as 12 days’ gestation. It appears to function early in pregnancy to protect the growing embryo and fetus from certain infectious organisms such as the spirochete of syphilis.

This layer of cells disappears, however, between the 20th and 24th weeks.

The placenta (

Latin for “pancake,” which is descriptive of its size and appearance at term) arises out of the continuing growth of trophoblast tissue. Its growth parallels that of the fetus, growing from a few identifiable cells at the beginning of pregnancy to an organ 15 to 20 cm in diameter and 2 to 3 cm in depth, covering about half the surface area of the internal uterus at term.

Placental osmosis

is so effective that all except a few substances are able to cross from the mother into the fetus. Because almost all drugs are able to cross into the fetal circulation, it is important that a woman take no nonessential drugs (including alcohol and nicotine) during pregnancy

Diffusion

When there is a greater concentration of a substance on one side of a semipermeable membrane than on the other, substances of correct molecular weight cross the membrane from the area of higher concentration to the area of lower concentration. Oxygen, carbon dioxide, sodium, and chloride cross the placenta by this method.

Facilitated diffusion

To ensure that a fetus receives sufficient concentrations of necessary nutrients, some substances cross the placenta guided by a carrier so move more rapidly or easily than would occur if only simple diffusion were operating. Glucose is an example of a substance that crosses by this process.

Active transport

This process requires the action of an enzyme to facilitate transport. Essential amino acids and water-soluble vitamins cross the placenta by this process. The process ensures that a fetus will have adequate amino acid concentrations for fetal growth.

Pinocytosis

This is absorption by the cellular membrane of microdroplets of plasma and dissolved substances. Gamma globulin, lipoproteins, and phospholipids all have molecular structures too large for diffusion so cross in this manner. Unfortunately, viruses that then infect the fetus can also cross in this manner.

Human Chorionic Gonadotropin

The first placental hormone produced, hCG, can be found in maternal blood and urine as early as the first missed menstrual period (shortly after implantation has occurred) through about the 100th day of pregnancy.

Because this is the hormone analyzed by pregnancy tests, a false-negative result from a pregnancy test may be obtained before or after this period.

Estrogen

(primarily estriol) is produced as a second product of the syncytial cells of the placenta.

contributes to the woman’s mammary gland development in preparation for lactation and stimulates uterine growth to accommodate the developing fetus.

Progesterone.

“hormone of mothers”

This is because, although estrogen influences a female appearance, progesterone is necessary to maintain the endometrial lining of the uterus during pregnancy. It is present in serum as early as the fourth week of pregnancy, as a result of the continuation of the corpus luteum.

This hormone also appears to reduce the contractility of the uterus during pregnancy, preventing premature labor.

Human Placental Lactogen (Human Chorionic Somatomammotropin).

is a hormone with both growth-promoting and lactogenic (milk-producing) properties.

It is produced by the placenta beginning as early as the sixth week of pregnancy, increasing to a peak level at term. It can be assayed in both maternal serum and urine.

It promotes mammary gland (breast) growth in preparation for lactation in the mother.

It also serves the important role of regulating maternal glucose, protein, and fat levels so that adequate amounts of these nutrients are always available to the fetus

The umbilical cord

is formed from the fetal membranes (amnion and chorion) and provides a circulatory pathway that connects the embryo to the chorionic villi of the placenta.

Its function is to transport oxygen and nutrients to the fetus from the placenta and to return waste products from the fetus to the placenta.

contains only one vein (carrying blood from the placental villi to the fetus) but two arteries (carrying blood from the fetus back to the placental villi).