The first trimester is weeks one through twelve, or the first three months. By the end of the first month, the embryo is the size of a grain of rice. It resembles a tadpole, with a little tail that eventually disappears. The heart, lungs, kidneys and the liver are beginning to form.
In the second month, the embryo's arms and legs start to grow, along with a defined torso and head. Eyelids, tongue, and teeth also start to form. The heart also starts to beat. At the end of this month, the embryo is now known as a fetus.
During the third month, that heartbeat can be detected, particularly around the 10-week mark. The tail disappears and fingers and toes start to form. Earlobes and eyelids continue to develop. Reproductive organs that determine whether the baby will be male or female are starting to form, but they cannot yet be detected on an ultrasound. The fetus has also developed a sense of touch, primarily in the cheeks, genitals, palms and soles of the feet. The fetus weighs about one ounce and is three inches in length.
The third month is also when the fetus can begin to suck its thumb, roll its fingers into a fist, and starts nodding its head.