metanephros, precursor of the definitive kidney, starts to develop. The initial stomach differentiation begins. Week 8 Gestational age: 7 weeks and 0 days until 7 weeks and 6 days old. 50–56 days from last menstruation. Embryonic age: Week nr 6. 5 weeks old. 36–42 days from fertilization. The embryo measures in length. Lungs begin to form. The brain continues to develop. Arms and legs have lengthened with foot and hand areas distinguishable. The hands and feet have digits, but may still be webbed. The gonadal ridge begins to be perceptible. The lymphatic system begins to develop. 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. Embryonic age: Week nr 7. 6 weeks old. 43–49 days from fertilization. The embryo measures in length. Fetal heart tone (the sound of the heart beat) can be heard using doppler. Nipples and hair follicles begin to form. Location of the elbows and toes are visible. Spontaneous limb movements may be detected by ultrasound. All essential organs have at least begun. The vitelline duct normally closes. Changes by weeks of gestation Weeks 10 to 12 Gestational age: 9 weeks and 0 days until 11 weeks and 6 days old. Embryonic age: 7 weeks and 0 days until 9 weeks and 6 days old. Embryo measures in length. Ventral and dorsal pancreatic buds fuse during the 8th week Intestines rotate. Facial features continue to develop. The eyelids are more developed. The external features of the ear begin to take their final shape. The head comprises nearly half of the fetus' size. The face is well formed. The eyelids close and will not reopen until about the 27th week. Tooth buds, which will form the baby teeth, appear. The limbs are long and thin. The fetus can make a fist with its fingers. Genitals appear well differentiated. Red blood cells are produced in the liver. Heartbeat can be detected by ultrasound. Weeks 13 to 16 Gestational age: 12 weeks and 0 days until 15 weeks and 6 days old. Embryonic age: 10 weeks and 0 days until 13 weeks and 6 days old. The fetus reaches a length of about . A fine hair called lanugo develops on the head. Fetal skin is almost transparent. More muscle tissue and bones have developed, and the bones become harder. The fetus