1st trimester of pregnancy, 1st trimester
The term “1st trimester” refers to the first stage of pregnancy. The 1st trimester begins with the first day of the last menstrual period and ends with the beginning of the 13th week of pregnancy (week 12 + 6).
Course of the 1st trimester
The first trimester begins before the actual pregnancy on the first day of the last menstrual period and continues until the 12th week of pregnancy. Based on the first day of the last menstrual period, an expected date of birth can be calculated within the first weeks of pregnancy. However, this provisional date of birth is only for orientation.
Due to the fact that many women have a rather irregular menstrual cycle, in which ovulation does not take place between the 12th and 14th day of the cycle, fertilisation of the egg can also take place later. Furthermore, by calculating the approximate week of pregnancy, important conclusions can be drawn as to whether the unborn child is developing in time. Most women notice that fertilisation has already taken place during the first trimester.
Above all, the typical pregnancy symptoms, such as pronounced fatigue and frequent vomiting, can be an early indication of pregnancy. However, since the first symptoms of pregnancy are often similar to normal premenstrual symptoms, not every woman immediately realises that a pregnancy exists. Only in the middle part of the 1st trimester can the absence of menstruation and a positive pregnancy test confirm the first suspicion.
The 1st trimester begins before the actual pregnancy begins with the first day of the last menstrual period. During the first week of this trimester, the egg matures and ovulates (approximately between the 12th and 14th day of the cycle). After ovulation, the mature egg cell remains fertile for a period of about 12 hours.
When the egg and sperm fuse together, the so-called “seedling” is formed, which is the later embryo. Immediately after successful fertilisation, the egg cell begins to divide several times. By the end of the third week of the first trimester, the fertilised egg has already divided several times and prepared for implantation.
The fertilised egg cell forms parts of the later placenta in addition to the child’s own systems. From the eighth day of development, the cells of the so-called “embryoblast” arrange themselves into three layers (germ layers) lying on top of each other. At this point, the outer (ectoderm) and inner (entoderm) cotyledons are formed.
In addition, a small cleft space, the so-called amniotic cavity, forms above the outer cotyledon. This amniotic cavity continues to expand during the first trimester of pregnancy and forms the inner part of the amniotic sac. While the cells of the outer cotyledon form the nervous system (brain, spinal cord, peripheral and central nervous system), sweat glands, enamel and nails, most of the internal organs are formed from the inner cotyledon.
Bones, muscles and blood vessels are formed from a cell layer between the ectoderm and the entoderm. Already within the first 4 weeks of the 1st trimester of pregnancy the development of the fetal heart begins. Around the sixth week of pregnancy, the heart activity of the unborn child can be detected by ultrasound.
The most important parts of the organism are already fully formed in the tenth week of the 1st trimester of pregnancy. In addition, in the first trimester of pregnancy all organs of the child are already in place. The ears, eyes and eyelids are also formed by the end of the 12th week of pregnancy. On average, by the end of the 1st trimester the fetus reaches a length of nine centimetres (from crown to rump) and a weight of about 40 to 50 grams.