Fetal Development and Circulation; Second and Third Trimester Mark A. Fischione, M.D. Pathological Sciences School of Osteopathic Medicine in Arizona

Highlights of Second Trimester Development The embryo is now officially known as a fetus It is the trimester of organ and tissue growth The weight of the fetus will multiply more than seven times over the next few months, as the fetus becomes a baby that can survive outside the uterus By the end of the second trimester, the fetus will be about 13-16 inches long and weigh about 2-3 pounds

Fetal Development in 2nd Trimester The following fetal development occurs in the second trimester: 1. The fetus moves, kicks, and can turn from side to side 2. The eyes have been gradually moving to the front of the face and the ears have moved from the neck to the sides of the head 3. The fetus can hear the mothers voice

Fetal Development in the 2nd Trimester 4. A creamy, white substance, called vernix caseosa begins to appear on the fetus and helps to protect the thin fetal skin and is gradually absorbed by the skin. It may be seen on the baby after birth 5. The fetus is developing reflexes such as sucking and swallowing and may respond to certain stimuli 6. The placenta is now fully developed

Vernix Caseosa

Vernix Caseosa

Thumb Sucking

Fetal Development in the 2nd Trimester 7. The brain undergoes its most important period of growth from the 5th month on. 8. Fingernails have grown on the tips of the fingers and toes, and the fingers and toes are fully separated. The skin is wrinkly and red, covered by soft, downy hair called lunugo hair, and hair is growing on the head 9. The fetus undergoes cycles of sleep and wakefulness

Lanugo Hair

Fetal Development in the 2nd Trimester 10. Fat begins to form on the fetus, and eyelids are beginning to open with visible eyebrows and eyelashes. 11. Fingerprints and toeprints have formed 12. The 20th week marks the halfway point of the pregnancy, and a fetus born at the end of the 24th week may survive in a neonatal intensive care unit

Fingernails and Eybrows

Third Trimester of Pregnancy Lasts from 28 weeks until the birth, usually between the 38th and 42nd weeks of pregnancy During this time, the fetus grows larger and the body organs mature The fetus moves frequently early on, but in the last two months, the baby is too big to move around inside the uterus. At the end of the third trimester, the fetus usually settles in a head down position in the uterus

Third Trimester of Pregnancy The fetus continues to gain weight and lengthen The lungs are maturing, still collapsed and atelectatic until the ninth month (around week 37) when adequate surfactant is produced by the Type II pneumocytes

Normal Placenta The usual term placenta is about 22-24 cm in diameter, and 2.5-3.0 cm thick. It generally weighs approximately 500 gms. (1lb.) The maternal surface is dark red or maroon in color and should be divided into lobules or cotyledons. It is important that this structure be complete with no missing cotyledons. The fetal surface should be shiny, gray and translucent enough to see the color of the underlying cotyledons. Fetal membranes (amnio-chorion) are usually gray, shiny, wrinkled and translucent.

Normal Placenta

Maternal Surface of Normal Term Placenta

Fetal Surface of Normal Term Placenta

Maternal Surface of Placenta (left) with Normal Fetus in Complete Amniotic Sac

The Placenta In early pregnancy, the Placental Membrane consists of the Syncytiotrophoblast, Cyto-trophoblast, connective tissue, and the endothelium of the fetal capillaries. In late pregnancy, the cytotrophoblast degenerates, and the connective tissue is displaced by the growth of the fetal capillaries, leaving the syncytiotrophoblast and the fetal capillary endothelium.

1st Trimester Chorionic Villi Placenta with Cell Layers (Cyto and Syncytiotrophoblast

Normal 2nd Trimester Placental Villi with Small Clusters of Syncytial Knots

Normal 3rd Trimester Placental-Small and Highly Vascularized Chorionic Villi

Fetal Circulation The fetal circulation differs from the adult -The fetal cardiovascular system is designed to serve the prenatal needs and is then modified at birth. It differs from that of the adult, as the lungs, kidneys, liver, and digestive tract do not function and need very little oxygen. The fetus derives its O2 and nutrients and eliminates its CO2 and wastes through the maternal blood via the placenta.

Fetal Circulation The placenta is attached to the navel of the fetus by the umbilical cord which communicates with the mother through countless small blood vessels that emerge from the walls of the uterus. The umbilical cord contains blood vessels that branch into capillaries within the placenta. Wastes from the fetal blood diffuse out of the capillaries, into the interstitial fluid of the placenta, and finally into maternal capillaries.

Fetal Circulation

Fetal Surface of Normal Term Placenta

Maternal Surface of Normal Term Placenta

Fetal Circulation

Fetal Circulation Nutrients travel in the opposite direction. Good respiration in the newborn infant is dependant upon circulatory changes occurring at birth. The process of “Fetal Circulation” begins with: Well-oxygenated blood enters into the mother’s uterus and placenta to enter into one Umbilical Vein, about 80% saturated with oxygen. The umbilical vein ascends to the fetal liver where it branches into two conduits:

Fetal Circulation

Fetal Circulation 1. A small portion of the blood passes in the Portal Sinuses (which will join the portal vein) and enters into the hepatic sinusoids to bathe the liver cells. 2. Most of the blood bypasses the liver by entering into the Ductus Venosus which connects directly with the inferior vena cava. After a short course in the inferior vena cava, the blood enters the heart through the right atrium.

Fetal Circulation

Normal Umbilical Vein (Connects to the Ductus Venosus and Portal Sinuses)

Fetal Circulation Because the inferior vena cava contains deoxygenated blood from the lower limbs, abdomen, and pelvis, the blood entering the right atrium is not so well oxygenated, as was in the umbilical vein (about 65% O2 saturation) Deoxygenated blood returning from the upper regions of the fetus is collected by the superior vena cava and it also passes into the right atrium.

Fetal Circulation

Fetal Circulation The blood does not go into the lungs from the heart? Unlike postnatal circulation, most of the blood does not pass into the right ventricle, and then the lungs. Because the fetal lungs do not operate, an extensive pulmonary circulation would be a wasted effort. The blood in the right atrium passes through an opening called the Foramen Ovale. Exists in the intra-atrial septum, and diverts blood into the systemic side of circulation.

Fetal Circulation

Fetal Circulation Some blood from the superior and inferior vena cava does manage to enter into the right ventricle, and the blood leaves the pulmonary trunk, but only about 10% of this blood enters the lungs to bathe these tissues. Most of the blood in the pulmonary trunk is sent through the Ductus Arteriosus, a small vessel connecting the pulmonary artery with the aorta, further enabling the blood to bypass the lungs.

Fetal Circulation

Fetal Circulation The blood that entered into the left atrium enters through the foramen ovale, and enters into the left ventricle and leaves via the ascending aorta. The vessels to the heart, head, neck, and upper limbs receive rather well-oxygenated blood. Most of the mixed blood in the descending aorta (about 50% saturated with O2) passes into the umbilical arteries and is returned to the placenta for reoxygenation.

Fetal Circulation

Fetal Circulation The remainder circulates through the lower part of the body. The umbilical arteries are branches of the two internal iliac arteries, which pass through the umbilical cord at the placenta. The blood picks up O2 from the mother and eliminates CO2 and wastes. The oxygenated blood once again enters the fetus by way of the umbilical vein. The only vessel which carries fully oxygenated blood in the Umbilical Vein.

Fetal Circulation

Normal Umbilical Arteries (and Vein) Which Pass into the Umbilical Cord

Changes in the Fetus After Birth There are significant changes after birth. The lungs, GI tract, and liver become functional. The foramen ovale, umbilical vessels, ductus arteriosus, and venosus are no longer needed. Aeration of the lungs causes a dramatic fall in the pulmonary vascular resistance, causing a marked increase in pulmonary blood flow. Pressure in the left atrium is raised above the pressure in the right atrium.

Changes in the Fetus After Birth

Changes in the Fetus After Birth Increased left atrial pressure causes the Foramen Ovale to close by pressing the valve of the Foramen Ovale (the Septum Primum) against the Septum Secundum. The Ductus Arteriosus constricts, atrophies, and becomes the Ligamentum Arteriosum. The Ductus Venosus becomes a fibrous cord in the liver called the Ligamentum Venosum.

Changes in the Fetus After Birth

Changes in the Fetus After Birth The Umbilical Arteries constrict, atrophy, and become the Medial Umbilical Ligaments. The intra-abdominal portion of the Umbilical Veins forms the Ligamentum Teres, which passes from the umbilicus to the undersurface of the liver, where it attaches to the left branch of the portal vein. It remains patent for some time and may be used for exchange transfusions during early infancy.(i.e.Erythroblastosis fetalis) The placenta is shed as the afterbirth.

Intra-abdominal Portion of Umbilical Vein may be used for Transfusions

Changes in the Fetus After Birth

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