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"Weeks of pregnancy" refers to the number of weeks after the first day of the mother's last menstruation. Thus, in the first two "weeks of pregnancy", the mother's oocyte hasn't even ovulated, let alone been fertilized.
Recall that when a sperm cell fertilizes an oocyte, the resulting cell is called a zygote. This cell will cleave several times to form many smaller cells. When the zygote has cleaved into 16 cells, it is a ball of cells called a morula (three to four days after fertilization/conception). The morula fills with fluid contained within a thin layer of cells called the trophoblast (this layer of cells will become the placenta); within the fluid is the inner cell mass, which becomes the embryo. It does so by first undergoing gastrulation—becoming the three germ layers (endoderm, mesoderm, and ectoderm). It is around this time (9-12 days after fertilization, or the fourth "week of pregnancy") that the embryo implants in the endometrium.
During weeks four through eight of pregnancy, the basic body plan of an embryo will begin to develop. During the process of gastrulation (when the three germ layers differentiate), an embryonic disk will form. This disk doesn't look much like a person, but it is essentially the first stage of setting up a body plan of the embryo. From there, the neural tube will form within the embryonic disk and will actually become the brain and spinal cord. The cells that form into the neural tube originate from within the ectoderm (the inner layer of germ cells).
Following this, other germ cell layers will also begin to specialize and give rise to certain body features, organs, and organ systems. By the end of week eight, the embryo will actually begin to resemble a human being.
During weeks four through eight, the reproductive structures will also begin to develop, depending on the inheritance of the sex chromosomes. Sex chromosomes are the chromosomes that determine the sex of the individual. If the individual inherited X and Y chromosomes, male reproductive structures would form. If the individual inherited X and X chromosomes, female reproductive structures would form. However, the baby isn't large enough, and the sexual features not developed enough, to see on an ultrasound until about week 20 of pregnancy.
At the end of week eight, the embryo is then called a fetus. It is considered an embryo prior to week eight, and by the end of week eight, it becomes a fetus.
During weeks four through eight, there is always a chance a miscarriage can occur. Miscarriages can also occur later in development, although it's much less common. A miscarriage is when an embryo or fetus will be spontaneously expelled from the uterus. The cause or causes of miscarriage can be many factors, but often it is a genetic abnormality. As the body plan is being laid out and as the embryo is starting to develop into a fetus, genetic abnormalities can cause improper development. The body, often times, will expel the abnormal fetus from the uterus because of these genetic abnormalities.
The second trimester of pregnancy starts at the fourth month and lasts until the end of the sixth month. During this time of fetal development, organ and organ systems are maturing, and fetal movement can be felt.
The third trimester is marked as the seventh month until birth, which on average is within the ninth month. During this time, organ and organ systems are continuing to mature, and the fetus is preparing for birth. Babies who are born before the third trimester will generally have a low survival rate because their organ systems are too underdeveloped. The time within the mother is very important for organ and organ systems to properly develop. Essentially, if the baby is born before the seventh month, the baby's organ systems aren't well enough developed to allow for survival outside of the uterus.
The circulatory system of a fetus is composed of temporary vessels and lungs that do not function while inside the uterus. While the fetus is within the uterus gas exchange is not occurring like it usually would. The lungs at that point are not functioning because the placenta is playing this role by delivering oxygen and removing carbon dioxide; the fetus is not actually breathing. Once the baby is born, the circulatory system becomes independent, and the lungs will begin gas exchange when the first breath is taken. While the fetus is within the mother, the circulatory system depends on the mother's circulation to undergo gas exchange vs. breathing air.
Investigate embryonic and fetal development in three dimensions using augmented reality (AR)!
