Chapter 3 Fetal Development

Cell Division Mitosis –Continuous process –Body grows, develops, and dead cells are replaced –Each daughter cell contains same number of chromosomes as parent cell—called diploid Meiosis –Reproductive cells undergo two sequential divisions –Number of chromosomes is 23 per cell with only one sex chromosome—called haploid –At fertilization, the new cell contains 23 chromosomes from the sperm and 23 chromosomes from the ova (46) –Traits from Mother & Father

Fertilization Takes place in the outer third of the fallopian tube, near the ovary Sperm can carry either an X or Y chromosome Male determines the gender of the fetus pH of female reproductive tract influences survival rate of the X- and Y-bearing sperm, including speed of motility XX results in female XY results in male fertilization_ bc

Tubal Transport of the Zygote Zygote is formed by union of sperm and ovum Transported through fallopian tube into uterus During transport, zygote undergoes rapid mitotic division Size of zygote does not increase, individual cells become smaller as they divide, then form a solid ball (known as a morula) Morula

Implantation of the Zygote Usually occurs in upper section of posterior uterine wall day 7-9

Development At the time of implantation all cells are identical Cell differentiation –Occurs after implantation –Special functions Chorion - placenta Amnion – “bag of water” 1000ml at delivery –Envelops and protects the embryo Yolk sac –Functions only during the embryotic life and initiates the production of RBCs, until the embryotic liver takes over Primary germ layers –Ectoderm, Mesoderm, Endoderm

Functions of Amniotic Fluid Maintains an even temperature Prevents the amniotic sac from adhering to the fetal skin Allows symmetrical growth of fetus Allows buoyancy and fetal movement Acts as a cushion to protect the fetus and umbilical cord from injury Does not lubricate – Is not an oil

Three Stages of Prenatal Development Zygote: cell formed by union of sperm and ovum Embryo: 2nd to 8th week of development Fetus: 9th week until birth Age of viability: 20 weeks of gestation but requires NICU care to survive

Prenatal Development Embryo at 3 weeks, 4 weeks, 8 weeks

Prenatal Development (cont.) Week 3: neural tube forms, primitive heart starts beating time of first missed period folic acid supplements can prevent most neural tube defects How much is recommended? What foods are high in folic acid? planned pregnancies and early prenatal care are vitally important!

Prenatal Development (cont.) Week 10 – external genitalia visible to ultrasound, earliest to hear fetal heart rate with doppler ( bpm)

Placenta Temporary organ for fetal respiration, nutrition, and excretion Produces four hormones –Progesterone –Estrogen –Human chorionic gonadotropin (hCG) –Human placental lactogen (hPL)

Progesterone Functions during pregnancy –Maintains uterine lining for implantation of the zygote –Reduces uterine contractions to prevent spontaneous abortion –Prepares the glands of the breasts for lactation –Stimulates testes to produce testosterone, which aids the male fetus in developing the reproductive tract

Estrogen Functions –Stimulates uterine growth –Increases the blood flow to uterine vessels –Stimulates development of the breast ducts to prepare for lactation Effects of estrogen, not related to pregnancy –Increased skin pigmentation “mask of pregnancy” –Vascular changes in the skin and mucous membranes of nose and mouth –Increased salivation

Human Chorionic Gonadotropin (hCG) Hormone that “signals” to the corupus luteum that conception has occured Causes the corpus luteum to persist and continue production of estrogen and progesterone to sustain pregnancy hCG is detectable in maternal blood as soon as implantation occurs (usually 7 to 9 days after fertilization) Basis for most pregnancy tests

Human Placental Lactogen (hPL) Also known as human chorionic somatomammotropin (hCS) hPL causes decreased insulin sensitivity and utilization of glucose by mother –Helps to make more glucose available to fetus to meet growth needs

Umbilical Cord Two arteries carry blood away from fetus One vein returns blood to the fetus Wharton’s jelly covers and cushions cord vessels The umbilical cord usually protrudes near the center of the placenta TAV (two arteries one vein) The woman’s name AVA, which stands for “Artery-Vein-Artery”

Fetal Circulatory Shunts Ductus venosus –diverts some blood away from the liver as it returns from the placenta –Blood goes to the liver through portal sinus and inferior vena cava Foramen ovale –diverts most blood from the right atrium directly to the left atrium, rather than circulating it to the lungs (fetus gets oxygen from the mother) Ductus arteriosus –diverts most blood from the pulmonary artery into the aorta

Circulation Before Birth Oxygenated blood enters the fetal body through the umbilical VEIN After circulating through the fetal body, blood containing waste products returns to the placenta through the umbilical ARTERIES

Circulation after Birth Foramen ovale –closes within 2 hours after birth –permanently by age 3 months Ductus arteriosus –closes within 15 hours –permanently in about 3 weeks Ductus venosus –closes functionally when cord is cut –permanently in about 1 week

Circulation After Birth What causes these changes to occur? –Fetal shunts are not needed after birth once the infant breathes and blood is circulated to the lungs –Fetal circulation is determined at birth

Multifetal Pregnancy Monozygotic from a single fertilized ovum (identical twins) – Develop from a single fertilized ovum – Genetically identical (same sex, look alike) – Any physical differences caused by environmental factors variations in the blood supply from placenta – Most begin to develop at the end of first week after fertilization – Results in 2 identical embryos, each with own amnion. They share chorion, placenta, some placental vessels. Dizygotic is from two separate fertilized ovum (fraternal twins) –May or may not be of same sex –Develop from 2 separate ova, fertilized by 2 sperm –Have 2 chorions, 2 amnions, 2 placentas, can sometimes fuse –Tend to repeat in families, increased % with age

Multifetal Pregnancy (cont.)

Baby Center Inside Pregnancy: Weeks 1 to 9 – Inside Pregnancy: Weeks 10 to 14 Inside Pregnancy: Weeks 15 to 20 Inside Pregnancy: Weeks 21 to 27 Inside Pregnancy: Weeks 28 to 37

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