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Initial sway, total sway, and sway velocity (see figure for description of variables) are significantly less during the third trimester than during the second trimester and when compared to non-pregnant women. These biomechanical characteristics are possible reasons why falls are more prevalent during the second trimester during pregnancy.
Many symptoms and discomforts of pregnancy, such as nausea and tender breasts, appear in the first trimester. [56] During the second trimester, most women feel more energized and put on weight as the symptoms of morning sickness subside. They begin to feel regular fetal movements, which can become strong and even disruptive. [citation needed]
The parts of the fetal brain that control movement will not fully form until late in the second trimester, and the first part of the third trimester. [7] Control of movement is limited at birth, and purposeful voluntary movements develop during the long period up until puberty. [ 8 ]
At the end of the second week of development, some cells of the trophoblast penetrate and form rounded columns into the syncytiotrophoblast. These columns are known as primary villi . At the same time, other migrating cells form into the exocoelomic cavity a new cavity named the secondary or definitive yolk sac, smaller than the primitive yolk sac.
The developing parathyroid glands are physiologically functional beginning in the second trimester. [citation needed] Studies in mice have shown that interfering with the HOX15 gene can cause parathyroid gland aplasia, which suggests the gene plays an important role in the development of the parathyroid gland. [9]
The first ten weeks of gestational age is the period of embryogenesis and together with the first three weeks of prenatal development make up the first trimester of pregnancy. From the 10th week of gestation (8th week of development), the developing embryo is called a fetus.
Main development of sex organs starts. Week 9. Gestational age: 8 weeks and 0 days until 8 weeks and 6 days old. 57–63 days from last menstruation.
The first trimester is from the last menstrual period through the 13th week, the second trimester is 14th–28/29th week, and the third trimester is 29/30th–42nd week. [6] Birth normally occurs at a gestational age of about 40 weeks, though it is common for births to occur from 37 to 42 weeks. [ 6 ]