 , while the remainder perfuses the hepatic sinusoids . - The IVC carries this mixed but relatively oxygen-rich blood to the right atrium . - From the right atrium , blood follows two main routes: Majority of oxygenated blood is shunted through the foramen ovale (interatrial opening) into the left atrium , bypassing pulmonary circulation. From there, it flows to the left ventricle , and is pumped into the ascending aorta , supplying the heart, brain, and upper body . - Deoxygenated blood from the superior vena cava enters the right atrium , passes into the right ventricle , and is pumped into the pulmonary trunk .Due to high pulmonary vascular resistance , most of this blood is diverted through the ductus arteriosus into the descending aorta , bypassing the lungs. - Blood from the descending aorta supplies the lower body , and deoxygenated blood is returned to the placenta via the paired umbilical arteries , which arise from the internal iliac arteries . Key adaptations: Foramen ovale, ductus venosus, ductus arteriosus Clinical relevance: These shunts must close at birth to transition to adult circulation. There are three key shunts in fetal circulation, each ensuring efficient distribution of oxygenated blood while bypassing non-functional fetal organs like the lungs and liver. - Ductus venosus Embryological origin : Derived from the vitelline venous system . - Function : Shunts oxygenated blood from the umbilical vein directly into the inferior vena cava , bypassing the hepatic circulation . - Fate : Closes shortly after birth and becomes the ligamentum venosum . - Foramen ovale Embryological origin : Formed between the septum primum and septum secundum in the interatrial septum. - Function : Allows blood to bypass the non-ventilated lungs by shunting blood from the right atrium to the left atrium . - Fate : Functionally closes at birth due to reversal of atrial pressure; anatomically forms the fossa ovalis . - Ductus arteriosus Embryological origin : Derived from the left sixth aortic arch . - Function : Connects the pulmonary trunk to the descending aorta , bypassing the lungs. - Fate : Closes in response to increased oxygen tension and decreased prostaglandins, becoming the ligamentum arteriosum . (i) Tetralogy of Fallot (TOF) - Components :Pulmonary stenosis - Overriding aorta - Ventricular septal defect (VSD) - Right ventricular hypertrophy - Embryological basis :Caused by anterior and superior displacement of the infundibular (conotruncal) septum , derived from neural crest cells . - This leads to unequal division of the truncus arteriosus , favoring the aortic outflow over the pulmonary, and malalignment of the interventricular septum . (ii) Patent Ductus Arteriosus (PDA) - Embryological basis :The ductus arteriosus , derived from the left sixth aortic arch , fails to close postnatally. - Persistence is often related to prematurity or conditions associated with low oxygen tension (e.g., respiratory distress). - Clinical consequence :Causes a left-to-right shunt , leading to pulmonary overcirculation and heart failure if untreated. Lung development involves both structural maturation and functional readiness for gas exchange , divided into five overlapping stages: - Embryonic stage (4–7 weeks) Origin : Lung bud (respiratory diverticulum) arises from the ventral foregut endoderm . - Bud bifurcates into primary bronchial buds , which invade the splanchnic mesoderm . - Pseudoglandular stage (5–17 weeks) Branching continues to form terminal bronchioles . - Histologically resembles an exocrine gland . - No gas exchange is possible. - Canalicular stage (16–25 weeks) Terminal bronchioles give rise to respiratory bronchioles and alveolar ducts . - Vascularization increases , and the epithelium starts to thin. - Type I pneumocytes begin forming. - Saccular stage (24–38 weeks) Formation of terminal sacs (primitive alveoli) . - Type II pneumocytes differentiate and start secreting surfactant , crucial for alveolar stability post-birth. - Alveolar stage (36 weeks to 8 years postnatally) Mature alveoli form via septation of terminal sacs. - Gas exchange becomes efficient , and lung capacity expands after birth. Clinical note : Premature infants born before surfactant production (~24–28 weeks) are at risk of neonatal respiratory distress syndrome (NRDS) . Oogenesis is the process by which female gametes (ova) are formed from primordial germ cells (PGCs) , beginning during fetal life and continuing until menopause. Stages of oogenesis: - Primordial germ cells (PGCs) migrate to the gonadal ridge (~4th week of embryogenesis). - They differentiate into oogonia via mitosis (~5th–6th week).Peak number (~6–7 million) is reached by mid-gestation (around 20 weeks). - Oogonia enter meiosis I and become primary oocytes , arrested in prophase I (diplotene stage) until puberty.Each is surrounded by flattened follicular cells, forming primordial follicles . - At puberty, under FSH stimulation , some follicles resume meiosis I during each menstrual cycle. Completion of meiosis I yields a secondary oocyte and a first polar body . - The secondary oocyte immediately begins meiosis II , but is arrested at metaphase II until fertilization. - Upon fertilization, meiosis II is completed , producing the ovum and a second polar body . Key features: - Oogenesis is a discontinuous process , with long arrest periods. - Only 400–500 oocytes are ovulated during a woman’s reproductive life. - Aging affects oocyte quality, increasing the risk of meiotic nondisjunction (e.g., trisomies). Placental abnormalities can be classified based on structural development and implantation site . A. Structural abnormalities: - Placenta accreta spectrum (PAS): Accreta : Villi attach directly to myometrium (defective decidua basalis). - Increta : Villi invade the myometrium . - Percreta : Villi penetrate through to the serosa or adjacent organs (e.g., bladder). - Cause : Often associated with previous C-sections or endometrial damage . - Circumvallate placenta :The chorionic plate is smaller than the basal plate, leading to a rolled, thickened edge . - May be associated with bleeding, preterm labor, and fetal growth restriction . - Succenturiate lobe :An accessory placental lobe , increasing the risk of retained placental tissue postpartum . B. Positional abnormalities: - Placenta previa :Implantation over or near the internal cervical os . - Presents with painless vaginal bleeding in late pregnancy. - Types: Total, partial, marginal. - Placenta abruptio : Premature separation of a normally implanted placenta. - Associated with painful bleeding and fetal distress. The placenta forms from both embryonic and maternal tissues , beginning in the second week of development . Fetal (embryonic) contribution : - From the trophoblast : Cytotrophoblast : Inner cellular layer; proliferative. - Syncytiotrophoblast : Outer multinucleated layer; invades endometrium and forms lacunae . - Extraembryonic mesoderm invades and forms the chorionic villi . Developmental stages : - Primary villi (Day 13) : Core of cytotrophoblast covered by syncytiotrophoblast. - Secondary villi (Day 16) : Extraembryonic mesoderm invades primary villi. - Tertiary villi (~Day 21) : Capillaries develop within mesodermal core. These villi become vascularized , allowing maternal–fetal exchange . Maternal contribution : - From the decidua basalis of the endometrium. - Forms the maternal side of the placenta with intervillous spaces filled with maternal blood. Mature placenta : - By the end of the first trimester, villi on the chorion frondosum (fetal side) persist. - The decidua ba