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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.
Lung bud, the first traits of the lung appear. [1] Hepatic plate, the first traits of the liver appear. [1] Buccopharyngeal membrane ruptures. This is the future mouth. [1] Cystic diverticulum, which will become the gallbladder, and dorsal pancreatic bud, which will become the pancreas appear. [1] Urorectal septum begins to form.
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 ]
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]
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]
The heart rate increases, but generally not above 100 beats/ minute. Total systematic vascular resistance decreases by 20% secondary to the vasodilatory effect of progesterone. Overall, the systolic and diastolic blood pressure drops 10–15 mm Hg in the first trimester and then returns to baseline in the second half of pregnancy. [6]