Ch 12 Conception and Prenatal Development

Conception and Prenatal Development Chapter 12 The fetal period is the 9th week after conception and ending with birth A few major developments occur at different gestational ages

We will go over the most common changes during these periods Weeks 13-16 Movements strengthen so woman who have been pregnant may begin to feel these movements during this period When a woman feels those movements it is called quickening During weeks 17-20 many women will feel fluttering or butterflies This is those first movements as well and this is the period that most first time moms will feel movement

Brown fat is produced during this period Brown fat is heat-producing fat deposits to help regulate temp Locations for brown fat include back of the neck, behind the sternum and around the kidneys Weeks 21-24 Baby is growing and the lungs produce surfactant Surfactant is a lipid that reduces surface tension in the alveoli in the lungs

They need this surfactant to keep the lungs from collapsing If born at the end of this period, they may survive but many that survive have multiple complications that can last their lifetime Weeks 25-28- Better chance of survival if born during this period The fetus will typically get in the head down position at the end of this period to get prepared for delivery Weeks 29-32 Pigmentation of skin according to race takes place during this time

Chances of survival are greater during this period Weeks 33-38- Growth continues until birth but will slow as it gets closer to full term Lanugo is fine hair that covers the body during earlier fetal periods and now during this time will decrease Vernix is the thick white substance that protects the skin It is in more abundance during the earlier fetal periods and not as abundant as the fetal reaches full term The placentas is the babies life-line It attaches to the uterus during implantation and the other side is where the umbilical cord attaches

It is a thick, disk-shaped organ The maternal component is the side that attaches to the uterus and provides nourishment to the fetus It may also help keep fetal tissue into entering into the maternal bloodstream Blood is circulated to and from the fetal side by the fetal heart The umbilical cord has 2 arteries and 1 vein

Remember AVA Artery, Vein, Artery A function of the placenta is metabolic function The placenta produces some nutrients needed by the fetus and synthesizes glycogen, cholesterol, and fatty acids Another function is the transfer of substances Gas exchange is a major function

The key function is respiration Oxygen and carbon dioxide pass through the placental membrane Another function is the passing of nutrients through the placenta from the mother The placenta also removes waste and it can transfer the antibody IgG from mom to the fetus via the placenta Endocrine function includes the ability of the placenta to produce many hormones needed for normal pregnancy

There are two fetal membranes: the amion and the chorion These membranes are so close that sometimes it is hard to tell there truly are two They are what is also known as the bag of waters The amnion is the inner membrane (closest to baby) and the chorion is the outer membrane which is closest to mom When moms “water breaks”, typically both membranes rupture

The amniotic fluid protects the fetus It protects by: cushioning the fetus against impact to the maternal abdomen, provides a stable temp It provides normal prenatal developing by allowing symmetrical development of the fetus, keeps the membranes from sticking to the fetal parts and provides room and buoyancy The amniotic fluid comes from fetal urine and fluid transported from the maternal blood across the amnion The amniotic fluid is an ongoing process

The fluid will cross the amion for absorption by the mom, some fluid is absorbed by the fetal lungs with breathing movements and it is swallowed an digested in the fetal digestive tract The volume increase during pregnancy and by full term is 500-1000mL Low amount of fluid is known as oligohydramnios and can be associated with poor fetal lung development and possible malformations It may occur because of poor kidney function to and they cannot excrete the urine Too much fluid is known as hydramnios or polyhydramnios

This is when the level typically exceed 2000 mL Can be caused from severe malformation of the CNS or GI tract which prohibits normal ingestion of the amniotic fluid There will be a video on Moodle for fetal circulation for you to review Your book also has good pictures explaining as well The umbilical cord has two arteries that carry blood high in carbon dioxide and other waste away from the fetus to the placenta

It is then transported to the mother’s circulation where she can excrete it The vein in the umbilical cord takes that nicely oxygenated blood along with the good nutrients back to the fetus These arteries and veins are coiled in the umbilical cord which allows for them to stretch Whartons jelly is a soft substance that encases the cord to prevent it from collapsing and becoming suppressed by pressure The fetal circulatory circuit needs to change after delivery

While in utero, the fetus does not breathe air or metabolizes substances There are 3 shunts that bypass most circulating blood away from the lungs and liver The first shunt is the ductus venosus and this is where about half of the oxygenated blood bypasses the liver and enters the vena cava This blood then enters the right atrium Most of the blood from here will enter the left atrium through the second shunt, the foramen ovale

This is where the blood mixes with a small amount of blood returning from the lungs The blood is then pumped from the left ventricle to the aorta so it can nourish the body A small amount of blood from the right ventricle is circulated to the lungs to nourish the lung tissue so it stays healthy for after delivery The remaining blood from the right ventricle joins the oxygenated blood from the aorta through the third shunt known as the ductus arteriosus The head and upper body will receive about 75% of this oxygenated blood

As pregnancy progresses into late pregnancy, the liver will now receive 75-80% of the oxygenated blood So after birth things need to change As the infant takes it first few breathes, these changes should occur First of all, with those first few breathes, the pressure from the blood flow increases and causes pressure on the right side to fall This drop in pressure causes the foramen ovale to close

Pressure in the aorta now increases and pressure in the pulmonary artery falls With this happening, the direction of blood flow through the ductus arteriosus is reversed from the aorta to the pulmonary artery The ductus arteriosus constricts as the arterial oxygen level rises The ductus venosus also constricts when the blood flow from the umbilical cord stops The foramen ovale and the ductus venosus permanently close and the ductus venosus and ductus arteriosus become ligaments along with the umbilical vein and arteries

Once again, the video on Moodle really gives you a nice visual of this all works There are two types of twins- monozygotic and dizygotic Mono are conceived of a single ovum and sperm that later divides into two They have identical gene make up so are known as identical twins Since they are identical- they must be the same sex

Most of the time they look identical but there can be some differences Mono twins happen randomly are not linked with any assisted reproductive technique (in vitro, etc) For most mono twins, they have two amnions but share the chorion If the division occurs earlier, there can be two separate sacs that implant separately These twins have separate placentas but often they may fuse together

In less common situations, the division happens later and they share their amnion and chorion So basically they are in the same bag of water Death can be more common in these twins because their cords can become tangled Remember the cord is the babies life line so it if becomes entangled, it cannot do its job to provide fetus with oxygen, nutrients and remove waste They can also have something that is called twin to twin transfusion and this is where one fetus will take all of the nutrients and the other gets very limited and also increases the risk of death in the smaller twin

Dizygotic twins are from two ova that are fertilized by two different sperm They are considered fraternal twins and can be same or opposite sex They may look alike or completely different They are no more alike than two siblings born at different times They can be hereditary

They have two separate sacs, their own membranes and placentas


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