However if two separate ova are released and fertilized the result can be fraternal twins. It could be two brothers, two sisters, or a brother and sister. They are no more genetically alike than any other two children from the same parents because they come from two separate eggs. Identical twins can result if one ovum is released and split into two parts. Identical twins genes are almost indistinguishable. After ovulation the first stage of the prenatal period begins, called the germinal stage which begins at conception and last through the second week of pregnancy.
About 2,000 sperm will reach the uterus and the fallopian tube. One of the sperm must attach itself to the ovum and penetrate its outer surface. The fertilized ovum, which is now called the zygote, will travel down the fallopian tube until it reaches the uterus. Then it attaches itself to the lining of the uterus and continues to grow. The second stage of the prenatal period is the embryonic stage. It spans the 2-8 weeks that follow conception. At this point the zygote is now known as the embryo.
The embryo begins to divide itself into two parts; one part will develop into the placenta while the other will go on to become your baby. Since all the basic organs are beginning to form, the embryo is very fragile. At about 21 days after conception, the beginnings of the spinal cord and eyes appear; at about 24 days, cells differentiate to form what will become part of the heart; at about 28 days, tiny buds appear that will develop into arms and legs; and at about 42 days, features of the face take shape. This is the stage where most major birth defects occur and when most miscarriages happen.
Toward the end of the embryonic stage, the organism has developed a number of body organs, such as the heart. The embryo is only about 4 cm long but already has the beginnings of major body organs and limbs to look some-what human. Structures important to the support of the embryo develop during the embryonic stage, including the placenta and umbilical cord. The placenta plays a vital role in the development of your baby. The placenta is an organ that connects the blood supply of the mother to that of the fetus.
The placenta feeds and nourishes the fetus while also disposing of toxic waste. Without it the baby could not survive. The third and final stage of the prenatal period is the fetal stage which begins two months after conception and lasts until birth. The fetus is becoming more of an individual during this stage. It develops vital organs such as lungs and physical characteristics. A fetuss eyes and eyelids are completely formed; they have a fine coating of hair, lungs that are beginning to function, and well developed external sex organs develop at about 6 months.
At the end of the fetal stage birth occurs and the fetus becomes a newborn. Sometimes babies are born before the 37th week of pregnancy, which are called premature babies. Because they are born too early, they weigh much less than a full-term baby. They may end up having health problems because their organs did not have enough time to develop. Premature babies need special medical care in a neonatal intensive care unit. They would stay in the NICU until their organ systems can work on their own.
Sadly some premature babies dont make it. A number of genetic errors can be tested for by amniocentesis during the fetal stage. Amniocentesis is a medical test done between weeks 14 and 20 of pregnancy. Amniocentesis consists of inserting a long needle through the mothers abdominal muscles into the amniotic fluid surrounding the fetus. Doctors can identify a number of genetic problems by withdrawing and analyzing fetal cells in the fluid. More than 1,000 genetic disorders can be tested for and identified.