13th week: 85-91 days:
By thirteen weeks, your baby may be able to put a thumb in her mouth, although the sucking muscles aren't completely developed yet. By the thirteenth week the eye is fully developed and the lid has shut. It will remain closed until the fetus is about seven-month gestation. At four months feet and toes are developed and the legs are beginning to take their natural proportions.
Recent studies using electron microscopy and immunocytochemical methods show that the development of various types of cells in the dorsal horn (along with their laminar arrangement, synaptic interconnections, and specific neurotransmitter vesicles) begins before 13 to 14 weeks of gestation and is completed by 30 weeks
14th week: 92-98 days
The fetus is now 3 inches long and weighs almost an ounce. Muscles lengthen and become organized at this point. Fist signs of movement are becoming evident. The arms, legs, knees and elbows will be move constantly. The hand will find the mouth and sucking may be seen. The hand close-up will seem perfectly formed although they are translucent and cartilage is still visible.
The Epidermis (outer skin) has now replaced the first protective membrane. The heart is well formed, it beats rapidly at 140 pulsations a minute.
The fetus has an adult's taste buds and may be able to savor the mother's meals.
Although the baby's first muscle movements were involuntary, the first voluntary muscle movements occur around week 16. After this point, awake or asleep, the baby moves 50 times or more each hour, flexing and extending her body, moving her head, face, and limbs, and exploring her warm, wet compartment by touch. A baby may touch her face, touch one hand to the other hand, clasp her feet, touch her foot to her leg, or her hand to the umbilical cord
By the end of the Fourth month, the baby will be eight to ten inches long and could weigh one pound or more. The baby will move around inside the sac seeking comfortable positions. Bone marrow is also forming at this stage, the heart can be plainly heard and is pumping up to twenty-five quarts of blood a day.
17th week:113-119 days:
The nails on the hand have grown by the seventeenth week and the forearm and upper arm have lengthened. Between the fifth and sixth month, the baby will be able to fully grasp the umbilical cord, and will recognize sounds and voices. Oil and sweat glands now function. The entire body is covered by fine hair. Vernis, Latin for “varnish” covers the baby in a protective layer. This white greasy substance will stay on the baby until it is born. Baby’s born premature, can survive outside of the womb at this point.
18th week: 120-126 days
The fetus is now about 5 inches long. The child can blink, grasp, and move her mouth. Hair grows on the head and body. The nerve tissue begins to mature. The fetus is growing rapidly and only has room to move when the mother is at rest. When the mother moves, the fetus will curl into a self-protective position. The mother may begin to feel the life now growing within her. As early as week 18, when the eyes are still closed, a baby's retinas can detect a small amount of light filtering through a mother's tissue if she's out in the bright sun or under strong lights.
20th week:134-140 days
Cutaneous sensory receptors spread to all cutaneous and mucous surfaces by the 20th week
Twins at 20 weeks' gestation can be seen developing certain gestures and habits that persist into their postnatal years. In one case, a brother and sister were seen playing cheek-to-cheek on either side of the dividing membrane. At one year of age, their favorite game was to take positions on opposite sides of a curtain, and begin to laugh and giggle as they touched each other and played through the curtain.
22nd week:148-154 days
23rd week:155-161 days
24th week:162-168 days
By the end of the second trimester it can hear.