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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]
Prenatal development (from Latin natalis 'relating to birth') involves the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization , in the germinal stage of embryonic development, and continues in fetal development until birth .
During pregnancy the plasma volume increases by 40-50% and the red blood cell volume increases only by 20–30%. [22] These changes occur mostly in the second trimester and prior to 32 weeks gestation. [24] Due to dilution, the net result is a decrease in hematocrit or hemoglobin, which are measures of red blood cell concentration.
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]
Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization. The following table summarizes the various expression systems during week number x of gestation.
Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). [1] It is typical for mammals, but also occurs for some non-mammals. Mammals during pregnancy can have one or more gestations at the same time, for example in a multiple birth. [2]
At 20 weeks, the fetus is able to implement feedback mechanisms for the production of thyroid hormones. During fetal development, T 4 is the major thyroid hormone being produced while triiodothyronine (T 3) and its inactive derivative, reverse T 3, are not detected until the third trimester. [2]
Additional energy is required during pregnancy due to the expansion of maternal tissues and stored to support fetal development. In the first trimester (blastogenesis and early embryonic stages), the mother experiences a minimal weight gain (approximately 0.5-2 kilograms), while the embryo weighs only 6 grams.