Lecture 7 Development of the Fetus

What marks the boundary between the embryonic period and the fetal period?

8 weeks (embryonic period is weeks 0-8; fetal period is week 8 to birth, week 38)

What characterizes the fetal period?

Maturation of tissues and organs, rapid growth of the body (length and weight), and slowing of head growth

What is the head-to-body ratio at 3rd month vs. 5th month vs. birth?

1:1 at 3rd month, 1:3 at 5th month, 1:4 at birth

What are the key features of an 11-week human fetus?

5-7 cm crown-rump length, swelling of the umbilical cord, well-developed digits, irregular skull contours, eyes ventrally located

What are the key features of a 12-week human fetus?

~8 cm crown-rump length, primary ossification centers formed, external genitalia recognizable, vascularization visible, retraction of herniated loops, muscular activity

What are the key features of an 18-week human fetus?

~15 cm crown-rump length, eyebrows and head hair visible, outer ears in their final position

What are the key features of a 28-week human fetus?

~27 cm crown-rump length, well-rounded contours, most organ systems functional (except respiratory and nervous systems), birth at 7 months gives ~90% survival chance

What is the primary site of nutrient and gas exchange between mother and fetus?

The placenta

What are the 5 main functions of the placenta?

Protection, nutrition, respiration, excretion, hormone production

What are the two components of the placenta, and their origins?

Fetal component (chorion), derived from the trophoblast and extraembryonic mesoderm; maternal component, derived from the uterine endometrium

How does the placenta accommodate the fetus's increasing nutrient demand?

By increasing the surface area between maternal and fetal components through development of chorionic villi

What structures are visible in the 9-day human blastocyst related to trophoblast?

Trophoblastic lacunae, syncytiotrophoblast, and cytotrophoblast

What forms by 12 days that surrounds the exocoelomic cavity (primitive yolk sac)?

Extraembryonic splanchnic mesoderm and extraembryonic somatic mesoderm, with endoderm cells lining the primitive yolk sac

What structures are seen by 13 days in the developing blastocyst?

Primary villi, oropharyngeal membrane, connecting stalk, secondary yolk sac, chorionic plate (extraembryonic somatic mesoderm), chorionic cavity

What is a primary villus composed of?

Syncytiotrophoblast (outer) and cytotrophoblast (inner), with no mesoderm core yet

What is a secondary villus composed of?

Syncytiotrophoblast, cytotrophoblast, and a mesoderm core

What is a tertiary villus (tertiary stem villus) composed of?

Syncytiotrophoblast, cytotrophoblast, mesoderm core, and villous capillaries (fetal blood vessels)

By what day do tertiary stem villi appear, compared to primary villi?

Primary villi appear by ~13 days; tertiary stem villi appear by ~21 days

What is the decidua basalis?

The portion of the endometrium deep to the implantation site, forming the maternal component of the placenta

What is the decidua capsularis?

The portion of the endometrium overlying/covering the implanted conceptus

What is the decidua parietalis?

The remaining uterine endometrium not associated with the implantation site

What is the chorion frondosum?

The portion of the chorion associated with the decidua basalis that develops elaborate villi and becomes the placenta

What is the chorion laeve?

The smooth portion of the chorion away from the decidua basalis, where villi degenerate

What happens to the decidua capsularis, chorion laeve, and amnion by the end of the 2nd/3rd month?

They fuse together with the decidua parietalis as the amniotic cavity expands and obliterates the chorionic cavity

What structure ultimately becomes "the placenta" by the end of the 3rd month?

The chorion frondosum together with the decidua basalis

What is a birth defect also called?

Congenital malformation or congenital anomaly

How is a birth defect defined?

A structural, behavioral, functional, or metabolic disorder present at birth

What is the leading cause of infant mortality?

Birth defects

What percentage of birth defects are caused by genetic factors?

Approximately 28%

What percentage of birth defects are caused by environmental factors?

Approximately 3-4%

What percentage of birth defects are caused by multifactorial inheritance?

Approximately 20-25%

In what percentage of individuals with birth defects is the cause unknown?

40-45%

What are examples of environmental factors that can cause birth defects?

Infectious agents, radiation, chemical agents, nutritional deficiencies, maternal disease

When during gestation is the risk of birth defects being induced the highest?

During the embryonic period, peaking around week 5

What historical event linked limb defects to a drug crossing the placenta, and when?

In 1961, the sedative thalidomide was first linked to limb defects, providing evidence that drugs could cross the placenta and cause birth defects

What historical discovery in 1941 linked a maternal infection to fetal abnormalities?

Discovery that German measles (rubella) affecting a mother during early pregnancy caused cataracts and heart defects in the embryo

What is the estimated national prevalence of cleft lip with/without cleft palate (per 10,000 live births)?

10.63 per 10,000 live births (about 1 in 940)

What is the estimated national prevalence of Trisomy 21 (Down syndrome)?

14.47 per 10,000 live births (about 1 in 691) - the most common of the chromosomal anomalies listed

What is ultrasonography used for in prenatal diagnosis?

A noninvasive technique using high-frequency sound waves to create images; assesses overall fetal growth, development, and birth defects

When is ultrasonography typically performed, and what are the two approaches?

Performed between 18-20 weeks; can be transabdominal or transvaginal (transvaginal gives higher image resolution)

What is maternal serum screening used to measure?

Concentrations of serum alpha-fetoprotein (AFP), often combined with unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A

Where is AFP produced, and when does it peak?

Produced by the fetal liver; peaks at approximately 14 weeks, then passes into maternal circulation via the placenta

When is maternal serum screening typically performed?

Between 15-20 weeks of gestation

How do AFP, uE3, hCG, and inhibin A change in neural tube defects?

AFP is increased; uE3, hCG, and inhibin A are unchanged

How do AFP, uE3, hCG, and inhibin A change in Down syndrome?

AFP is decreased, uE3 is decreased, hCG is increased, inhibin A is increased

How do AFP, uE3, and hCG change in Trisomy 18?

All three (AFP, uE3, hCG) are decreased

What is amniocentesis, and how much fluid is withdrawn?

A procedure withdrawing approximately 20-30 mL of fluid from the amniotic cavity

When and how is amniocentesis performed?

Transabdominally, during the second trimester (between 15-20 weeks)

What can amniotic fluid be analyzed for in amniocentesis?

Biochemical factors like AFP; fetal cells for karyotyping (results take several weeks); PCR can test for specific genetic diseases

What is Chorionic Villus Sampling (CVS)?

A procedure collecting approximately 5-30 mg of villus tissue for genetic/chromosomal testing

When and how is CVS performed?

Transabdominally or transcervically, between 10-12 weeks of gestation

What is a key limitation of CVS compared to amniocentesis?

CVS does not assess neural tube defects or other structural malformations, though it detects genetic disorders and chromosomal anomalies faster (results in a few days)