Fertilization occurs when the head of a sperm penetrates a mature ovum. After penetration, the nuclei (which contain the genetic material) of the sperm and ovum fuse and the body and tail of the sperm drop off. The newly fertilized ovum cal
led a zygote then forms an outer layer that is impenetrable to other sperm. The zygote undergoes repeated cell divisions as it passes down the Fallopian tube. By the time it reaches the uterus, it has grown into a solid ball of cells called a morula. It then develops an inner cavity with a small cluster of cells to one side, this is called a blastocyst. The blastocyst implants in the wall of the uterus, at this point it is known as an embryo.
Ovulation occurs halfway through the menstrual cycle (13-16) before the start of a period, after which the released ovum is available for fertilization for about two days. Sperm can also live for about two days, so the peak fertile period is about four days (two per and two post-ovulation).
Blood circulation in the fetus is different from the normal circulation after birth. The fetus neither breathes nor eats, so oxygen and nutrients are obtained, via the placenta and umbilical cord, from the mother’s blood. The other fundamental difference in circulation is that blood bypasses the lungs in the fetus.
Fetal distress sometimes occurs as a temporary episode, but, if acidosis is severe, the distressed fetus.
FETAL HEART CIRCULATION
In the fetus, blood passes directly from the right atrium of the heart to the left atrium through the foramen ovale. Another channel, the ductus arteriosus, allow blood to pass from the pulmonary artery to the aorta. After both channel close, redirecting blood through the lungs.