Chapter 8: Third Month to Birth: Fetus & Placenta

Third Month to Birth: Fetus & Placenta details the methodologies for estimating fetal age using crown-rump length (CRL) and crown-heel length (CHL), noting significant developmental milestones such as the humanization of facial features, limb elongation, and the external differentiation of genitalia by the twelfth week. The text explores the physiological changes occurring through the second and third trimesters, including the appearance of lanugo, vernix caseosa, and the critical deposition of subcutaneous fat required for thermal regulation. Clinical distinctions are drawn between Low Birth Weight (LBW), Intrauterine Growth Restriction (IUGR), and Small for Gestational Age (SGA), highlighting the role of Insulin-like Growth Factor 1 (IGF-1) in fetal growth and the potential long-term metabolic consequences described by the Barker hypothesis. A significant portion of the chapter is dedicated to the embryological development and functional complexity of the placenta and fetal membranes. It explains the transformation of the trophoblast, the remodeling of maternal spiral arteries by cytotrophoblast invasion to create a low-resistance vascular system, and the structural differentiation into the chorion frondosum and decidua basalis. The hemodynamics of the intervillous spaces within placental cotyledons are described, defining the human placenta as hemochorial. Key placental functions are elaborated upon, including the exchange of gases and nutrients, the production of essential hormones like human chorionic gonadotropin (hCG), progesterone, estriol, and somatomammotropin, and the transmission of maternal IgG antibodies for passive immunity. The summary also covers pathologies such as preeclampsia, placental insufficiency, and hemolytic disease of the newborn (erythroblastosis fetalis) resulting from Rh incompatibility. Furthermore, the dynamics of amniotic fluid production and resorption are discussed, relating volume abnormalities like polyhydramnios and oligohydramnios to congenital defects. The formation and composition of the umbilical cord, containing Wharton jelly and umbilical vessels, are outlined. Finally, the chapter categorizes multiple gestations, differentiating between dizygotic (fraternal) and monozygotic (identical) twins based on zygosity and the timing of cleavage, which dictates the arrangement of chorionic and amniotic sacs, and concludes with an overview of the three stages of parturition.