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A pregnancy occurs when a sperm cell (sperm) fertilizes an egg (oocyte). This usually happens in the fallopian tube. The prerequisite for this is that a healthy egg cell is expelled from the ovary (ovulation) and taken up by the fallopian tube. In addition, healthy sperm cells have to penetrate the cervix into the uterus and further into the fallopian tubes. When the sperm cell and egg cell fuse in the fallopian tube, a new organism (zygote) is created.
The first few weeks that follow are decisive for whether the fertilized egg is properly implanted in the uterus and the embryo begins to develop.
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- Fertilization and implantation
- The embryo develops
- Hormonal changes
Fertilization and implantation
The female egg cell is the largest cell in the human organism, the male sperm cell is one of the smallest cells in the human organism. They are called germ cells and, in contrast to all other cells in the body, each have only one single set of chromosomes, each with 23 chromosomes. The genetic information is only complete when the cell nuclei of an egg and a sperm cell fuse: The newly created cell (zygote) contains 46 chromosomes, a double set of chromosomes, i.e. the genetic information of mother and father. A person can emerge from this in innumerable developmental steps.
Note An egg cell can only be fertilized for a few hours to a maximum of one day after ovulation. However, sperm can survive in the uterus or fallopian tube for up to three days after intercourse. This means that pregnancy can occur not only on the exact day of ovulation, but also if sexual intercourse takes place a few days before or a day after (fertile days).
After fertilization, the zygote slowly migrates in the fallopian tube towards the uterus. On the way there it begins to divide, and a germinal vesicle (blastocyst) with several cell nuclei and layers develops, from which organs, bones, muscles and other tissues later develop. The blastocyst finally implants in the uterus about six days after fertilization (implantation).
The embryo develops
In the next few days and weeks, the blastocyst undergoes numerous stages of division and development. Some of the cells develop into the actual embryo, others into the placenta. The amniotic sac and amniotic fluid also begin to form.
The first trimester of pregnancy is a crucial phase for the development of the unborn child: the facilities for all organs of the embryo are created, the heart begins to beat, and the limbs are trained. Even the smallest disturbances during this period can result in malformations or developmental disorders. If these are so severe that they are incompatible with life, a miscarriage occurs; one speaks of the “all or nothing principle”. The risk of congenital malformations is highest in the first ten weeks of pregnancy (embryonic period). The causes for this are, for example, infectious diseases, drugs or chemical substances.
At the end of the tenth week of pregnancy, the embryonic phase is completed, which is called a fetus. The organ systems are in place, but they continue to develop until the end of pregnancy.
More on the topic: This is how an unborn child develops
A woman's hormonal balance goes through major changes during pregnancy. Hormones help maintain pregnancy and create optimal conditions for the unborn child. For example, estrogens and progesterone, which are mainly formed in the ovaries and placenta, are important. The two hormones hCG and hPL are also produced in the placenta.
The estrogens in the woman's body ensure, among other things, increasing elasticity of the connective tissue, control the growth of the uterus and have important functions in the course of childbirth. Progesterone is essential in maintaining pregnancy in the early stages. It prepares the lining of the uterus for the fertilized egg cell to implant and, among other things, ensures improved blood flow and relaxation of the uterus. This prevents the uterus from rejecting the fertilized egg again. The progesterone level in the blood rises sharply in the first weeks of pregnancy and increases continuously until the end of pregnancy.
Just a few days after fertilization, the blastocyst begins to produce the pregnancy hormone hCG (human chorionic gonadotropin). Its concentration increases continuously until the end of the first trimester of pregnancy. Once it is high enough, pregnancy tests can show the hormone in the blood or urine. This is usually the case from the point in time when the next menstrual period would be expected. The highest hCG concentration is reached in the 10th to 12th week after fertilization. The main effect of hCG is to increase the production of estrogen and progesterone, thus helping to maintain pregnancy.
The hormone hPL (human placental actogen) is also produced in the placenta. Its concentration increases continuously until the end of pregnancy. It has growth hormone-like effects, causes the maternal blood sugar level to rise and supports the release of fatty acids to supply the embryo. It also promotes milk production in women.
The period of hormonal adjustment during the first seven to eight weeks shows whether the fertilized egg is properly implanted in the uterus and whether the embryo is developing further. Bleeding in early pregnancy can be a sign that hormone production in the body has not yet fully adapted to the pregnancy.