Week 12 Week 12, or 10 weeks after conception, is generally considered the last week of the first trimester. The fetus has developed fingernails and toenails. The corpus callosum, which connects the right and left sides of the brain, has begun to develop. The intestines are moving from the umbilical cord into the abdomen. The fetus now yawns. It is nearly 3 inches long and weighs about half an ounce.