Week Number 15
Blood vessels can easily be seen through the skin, which is very thin and translucent. The legs are longer than the arms.
Week Number 16
The fetus can make a fist, open its mouth, move its lips, and swallow. The fetus might suck its thumb. Fine lanugo hairs can be seen on the head. The nails are well formed. The heart is beating 117 to 157 times per minute. The stomach is producing digestive juices. The kidneys are producing urine.
Week Number 17
Brown fat begins to form. The rapid growth rate begins to slow down. White fatty material begins to enclose the nerve fibers of the spinal cord. Between the 17th and 20th weeks hearing develops; the fetus can hear internal-organ and outside-world sounds.
Week Number 18
The skeleton can be clearly outlined on x-rays of the fetus. Ears stand out from the head. Lanugo covers the entire body.
