Fetal DevelopmentFetal Development

RC 290

The PlacentaThe Placenta

Placental StructurePlacental Structure

Purple and pancake shaped

6 inch diameter, 1 inch thick

13-16 square meter surface area

Attaches to upper 1/3 of uterus

Maternal side: Decidua Basalis

Fetal side: Chorion (attaches to amniotic membrane)

Placental FunctionsPlacental Functions

Primary function is to act as organ of respiration and to be the pathway for exchange of nutrients and waste products between mother and fetus

Physical protection Hormone synthesis Iron and Glycogen storage Some metabolic functions Antibodies

Maternal-Fetal CirculationMaternal-Fetal Circulation

Note: Maternal and fetal blood do not come into direct contact!

Maternal-Fetal CirculationMaternal-Fetal Circulation

Blood comes to IV spaces (maternal side) from uterine arteries

Blood in villi (fetal side) comes from umbilical arteries

After exchange of O2 and CO2, the “fresh blood” returns to the fetus via the umbilical vein

Umbilical CordUmbilical Cord

2 arteries 1 vein Surrounded by

Wharton’s jelly

Amniotic SacAmniotic Sac

The outer layer of the umbilical cord forms the amniotic sac around the fetus

Fetus is in amniotic sac containing amniotic fluid– Amniotic fluid made up of maternal serum and fetal

urine and fetal lung fluid

3 Functions of amniotic fluid:– Shock absorption, temperature stability, sterile

environment

Blood Gas ValuesBlood Gas Values

Hence, fetal PO2 is 25-30 mmhg!

Why is fetal PO2 low?Why is fetal PO2 low?

Placental O2 consumption Admixture and shunting

– Like the lung, the IV space and the villi are not perfectly matched

Fetal hemoglobin (HbF)– More of it (when compared to adult blood) and has greater

affinity for O2– Concentration is 17-18 grams%– Made up of alpha and gamma chains of amino acids– HbF is present for up to two months after birth

Fetal O2 consumption is about twice what an adults is

Fetal CirculationFetal Circulation

OOOPS! This is fecal circulation!

Fetal CirculationFetal Circulation

Fetal CirculationFetal Circulation UV goes into liver and portal

circulation Most of blood shunts across

the Ductus Venosus into the inferior vena cava

As inferior vena cava empties into right atrium, most blood shunts across the Foramen Ovale into the left atrium where it enters the systemic circulation from the left ventricle

– The heart and brain get the best blood first!

Blood from superior vena cava goes into right ventricle and then to pulmonary artery

Most of this blood flow shunts across the Ductus Arteriosus into the aorta

– DA is in the descending aorta after the arch

Pre-ductal blood may have a higher PO2!

– Hence, only 3-10% of fetal cardiac out put perfuses the fetal lings

Summary: Fetal CirculationSummary: Fetal Circulation

3 Shunts: Ductus Venosus, Foramen Ovale, and Ductus Arteriosus

Placental CirculationOnly 3-10% of fetal cardiac output actually

perfuses the fetal lungs

Fetal Lung DevelopmentFetal Lung Development

Lung buds appear at 3.5 weeksTracheo-bronchial tree formed by 16 weeksAlveoli start forming at 20-24 weeks

– Surfactant also starts to formCapillary networks starts forming and

approaching alveoli at 26-28 weeksLungs and surfactant mature at 35 weeksNormal term is 40 weeks

SurfactantSurfactant

Reduces surface tension so alveoli don’t collapse with each exhalation

Is a phospolipid made up of dipalmityl-lecithin and sphingomyelin

Produced through two enzymatic pathways:– Methyl transferase– Phosphocholine

transferase This is the major

pathway and has a longer half-life

Matures last

Respiratory Problems at Birth!Respiratory Problems at Birth!

Immature lung architecture if born premature

And/or:Surfactant levels may not be adequate if:

– Premature birth– Production inhibited if significant hypoxia,

hypothermia, or hypoglycemia