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In humans, the fetal stage commences at the beginning of the ninth week, fertilisation age or eleventh week gestational age. At the start of the fetal stage, the fetus is typically about in length from crown-rump, and weighs about 8 grams. The head makes up nearly half of the size of the fetus. Breathing-like movements of the fetus are necessary for the stimulation of lung development, rather than for obtaining oxygen. The heart, hands, feet, brain and other organs are present, but are only at the beginning of development and have minimal operation. The genitalia of the fetus starts to form and placenta becomes fully functional during week 9.
week 9