Circulatory system
Before birth
The heart and blood vessels of the circulatory system, form relatively early during embryonic development, but continue to grow and develop in complexity in the growing fetus. A functional circulatory system is a biological necessity, since mammalian tissues can not grow more than a few cell layers thick without an active blood supply. The prenatal circulation of blood is different from postnatal circulation, mainly because the lungs are not in use. The fetus obtains oxygen and nutrients from the mother through the placenta and the umbilical cord.Blood from the placenta is carried to the fetus by the umbilical vein. About half of this enters the fetal ductus venosus and is carried to the inferior vena cava, while the other half enters the liver proper from the inferior border of the liver. The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein. The blood then moves to the right atrium of the heart. In the fetus, there is an opening between the right and left atrium (the foramen ovale), and most of the blood flows from the right into the left atrium, thus bypassing pulmonary circulation. The majority of blood flow is into the left ventricle from where it is pumped through the aorta into the body.
Microcephaly (from New Latin microcephalia, from Ancient Greek μικρός mikrós "small" and κεφαλή kephalé "head") is a medical condition involving a smaller-than-normal head. Microcephaly may be present at birth or it may develop in the first few years of life. Since brain growth is correlated with head growth, people with this disorder often have an intellectual disability, poor motor function, poor speech, abnormal facial features, seizures and dwarfism.The disorder is caused by a disruption to the genetic processes that form the brain early in pregnancy, though the cause is not identified in most cases. Many genetic syndromes can result in microcephaly, including chromosomal and single-gene conditions, though almost always in combination with other symptoms. Mutations that result solely in microcephaly (primary microcephaly) exist but are less common. External toxins to the embryo, such as alcohol during pregnancy or vertically transmitted infections, can also result in microcephaly. Microcephaly serves as an important neurological indication or warning sign, but no uniformity exists in its definition. It is usually defined as a head circumference (HC) more than two standard deviations below the mean for age and sex. Some academics advocate defining it as head circumference more than three standard deviations below the mean for the age and sex.There is no specific treatment that returns the head size to normal. In general, life expectancy for individuals with microcephaly is reduced, and the prognosis for normal brain function is poor. Occasional cases develop normal intelligence and grow normally (apart from persistently small head circumference).