Doctors calculate pregnancy from the last menstrual period, so at eight weeks of pregnancy, the baby is six weeks from conception. In the first eight weeks of development, the baby is technically called an embryo, and after that it is a fetus. By the eighth week of pregnancy, almost all of the baby’s major organs have formed.
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Doctors can visualize the baby in the uterus using ultrasound technology. Similar to sonar, ultrasound generates an image by interpreting sound waves as they bounce off an object, in this case, the embryo. In early pregnancy, doctors might use transvaginal ultrasound, inserting the sound-wave emitting microphone into the vagina to locate the tiny embryo.
Using ultrasound, doctors can measure the length of the baby in the uterus to get a precise indication of gestational age and general health. During the first trimester of pregnancy, babies are measured in “crown-rump” length, or CRL, from the top or crown of the head to the tip of the hip bones used when sitting. By the eighth week of pregnancy, when the embryo is 6 weeks old, he measures from 1/2 inch to 3/4 inch long in crown-rump length. The authors of “Your Pregnancy Week by Week” say that the embryo is about the size of a pinto bean.
By the end of the eighth week, the embryo can make small, spontaneous movements that are visible on transvaginal ultrasound. According to “A Child is Born,” at eight weeks the embryo is active, seldom sleeping. At first, movements affect the whole body, but quickly the baby can move one part while the rest of the body stays still.
Compared to week seven, by week eight the baby’s trunk has lengthened and straightened. By the end of the eighth week, the embryo is recognizably human, with arms and legs, fingers and toes. The arms and legs are stubby projections, but the arms show a definite bend at the elbows and curve lightly over the chest. The beginnings of fingers and toes are present but have not separated from each other yet, and so are called digital rays and toe rays. The baby’s head is disproportionately large because during this stage of pregnancy the brain develops rapidly. Eyelid folds are forming and the nose has a distinct tip. Both the internal and external ears are forming.
By five to six weeks, ultrasound can detect the baby’s heartbeat, and by the eighth week, the aortic and pulmonary valves of the heart have formed. The breathing tubes from the throat to the lungs have branched out. The kidneys begin to produce urine. By eight weeks, the liver is visible on ultrasound as a bulge in the lower abdomen.