Fetal development

Week 1
The first week of human development begins with fertilization of the egg by sperm forming the first cell. After fertilization, the egg begins rapidly dividing into many cells. Then it is moving along the fallopian tube into the uterus for implantation in the uterine wall.

Week 2
The second week of human development is concerned with the process of implantation and the differentiation of the blastocyst into early embryonic and placental forming structures.

Week 3
The embryo is now made of three layers. The top layer — the ectoderm — will give rise to baby’s outermost layer of skin, central and peripheral nervous systems, eyes, and inner ears. Baby’s heart and a primitive circulatory system will form in the middle layer of cells — the mesoderm. This layer of cells will also serve as the foundation for baby’s bones, ligaments, kidneys and much of the reproductive system. The inner layer of cells — the endoderm — is where baby’s lungs and intestines will develop. The embryo starts folding.
At this week also early placenta is forming.

Week 4
The fourth week of embryonic development is the beginning of organogenesis.
Folding of the embryo continues and the earliest functioning organ is the heart. The neural tube along baby’s back is closing. The baby’s brain and spinal cord will develop from the neural tube. Other systems such as the circulatory, digestive and urogenital begin to all take shape.

Week 5
Five weeks after conception, baby’s brain and face are growing. Depressions that will give rise to nostrils become visible, and the beginnings of the retinas form. Lower limb buds that will become legs appear and the arm buds that sprouted last week now take on the shape of paddles. Heart septation starts and the heart is now divided into atrial and ventricular part.

Week 6
Six weeks after conception, baby’s lower limb buds take on the shape of paddles. Fingers have begun to form. Small swellings outlining the future shell-shaped parts of baby’s ears develop and the eyes become obvious. The upper lip and nose have formed. The trunk and neck begin to straighten. By the end of this week, baby might be about 11 to 14 millimeters long.

Week 7
Baby’s arms grow and elbows appear. Toes are visible and eyelids form. Baby’s head is large but still has a poorly formed chin. The part of the brain responsible for sense of smell is also taking shape. By the end of this week, baby might be a little less than 3/4 inch (16 to 18 millimeters) long— the diameter of a penny.

Week 8
In the 8th week of embryonic development baby’s head has become rounder. Baby can now bend his or her elbows. Toes and fingers lose their webbing and become longer. The eyelids and external ears continue to develop. The umbilical cord is clearly visible. By now, the heart has finished dividing into four chambers and the nose and sense of smell are developing.

Week 9
Baby is now officially described as a fetus. This week baby’s face is broad, the eyes widely separated, the eyelids fused and the ears low set. Buds for future teeth appear. Red blood cells are beginning to form in liver. By the end of this week, external genitalia will start developing into a penis or clitoris and labia majora. By now baby measures about 5 centimeters.

Week 10
10 weeks after conception, baby is sprouting fingernails. Baby’s face now has taken on a more developed profile. His or her intestines are in the abdomen. From now, the cartilage skeleton begins to develop into bone, starting with skull and long bones. The fetus is the size of a lime and weighs up to 14 grams.

Week 11
11 weeks after conception, the fetus is beginning to make urine and release it into the amniotic sac, making amniotic fluid. Skin is still thin and transparent, but it will start to thicken soon. Sex gland begins to form (ovaries or testes). Moreover, now the baby has discovered how to suck his or her thumb.

Week 12
Baby’s neck has become more defined and the lower limbs are well-developed. Red blood cells are forming in baby’s spleen. The baby’s sex now can be seen on ultrasound examination. Fine hair called lanugo is starting to appear on baby’s head and body. Baby is moving her arms and legs in a more coordinated way. Baby also is beginning to make slow eye movements under her closed lids.

Week 13
Baby measures nearly 10.1 centimeters from head to bottom and weighs about 70 grams. It is roughly the same size as an apple. Baby’s breathing and digestive systems are becoming more advanced. He’s using his swallowing and sucking abilities to breathe in amniotic fluid, which is good practice for his developing lungs. The bones of skeleton are harder and muscle tissue continues to develop.

Week 14
Baby’s neck muscles and back bones are now stronger, and head is more upright. Baby’s grasp now has developed enough to hold hands together. Baby’s scalp hair pattern also is forming.

Week 15
15 weeks after conception, toenails appear. Baby is becoming more active in the amniotic sac, rolling and flipping. His or her heart is pumping about 100 pints of blood each day. Fingerprints and toeprints continue to develop their unique swirls and patterns.

Week 16
Baby is about 14.2 centimeters from head to bottom and weighs up to 190 grams. Baby’s ears begin to stand out on the sides of his or her head. Baby might begin to hear. The eyes are beginning to face forward. Digestive system starts working.

Week 17
Baby’s arms and legs are now in the right proportion to the rest of his body, and thin hairs are starting to sprout from his scalp. His brain is forming separate, specialized areas that will be responsible for his sense of smell, taste, hearing, vision and touch. A greasy, cheese like coating called vernix caseosa begins to cover baby’s skin. The vernix caseosa helps protect delicate skin from abrasions, chapping and hardening that can result from exposure to amniotic fluid.

Week 18
18 weeks after conception, mother might be able to feel baby’s movements (quickening). Baby is regularly sleeping and waking. He or she might be awakened by noises or mother’s movements. This week, baby measures about 25.6 centimeters, or the length of a banana, and weighs about 300 grams.

Week 19
Baby is completely covered with a fine, downy hair called lanugo. The lanugo helps hold the vernix caseosa on the skin. The sucking reflex also is developing, enabling baby to suck his or her thumb. His skin is transparent and gradually turning from pink to red, as capillaries form underneath. Baby makes facial expressions, including mouthing, yawning, and sucking.

Week 20
20 weeks after conception, baby’s eyebrows and hair are visible. Brown fat also is forming, the site of heat production. For boys, the testes have begun to descend. Baby continues to swallow amniotic fluid, which is good practice for her digestive system. Her body is absorbing tiny amounts of sugar from the fluid, which supplements the nourishment she receives from the placenta.

Week 21
Baby now weighs the same as a large mango, which is a little more than 500 grams and measures about 29 centimeters from head to heel. Baby begins to have rapid eye movements. Baby is becoming more sensitive to sounds. He may have been able to hear mother’s heartbeat or voice as early as 16 weeks, but these sounds are now much clearer to him.

Week 22
In this stage, the eyelids begin to move and the eyes open. Patterns of sleeping and waking are becoming more defined. Baby’s brain is growing rapidly, and her facial muscles are getting a workout as she tests out different expressions, such as smiling and scowling.

Week 23
23 weeks after conception, baby might be able to respond to familiar sounds, such as mother’s voice, with movement. Baby is spending most of his or her sleep time in rapid eye movement (REM), when the eyes move rapidly even though the eyelids are closed. Baby now weighs nearly 660 grams and measures over 35 centimeters.

Week 24
24 weeks after conception, baby’s lungs continue to develop, branching out new airways with tiny air sacs (alveoli) at their tips. This network of airways is also known as the respiratory tree. Baby’s lungs are beginning to produce surfactant, the substance that allows the air sacs in the lungs to inflate and keeps them from collapsing and sticking together when they deflate.

Week 25
This week marks the end of the second trimester. 25 weeks after conception, nervous system is continuing to mature. Baby is also gaining fat, which will help his or her skin look smoother.

Week 26
26 weeks after conception, baby’s eyelids can partially open and eyelashes have formed. The central nervous system can direct rhythmic breathing movements and control body temperature. Baby weighs just over 1 kilogram. White fatty substance called myelin is slowly wrapping around nerves which in future will be vital for proper nerve conduction.

Week 27
Baby continues to mature and develop reserves of body fat. Baby is kicking more. Brain is developing rapidly at this time, and baby can see and hear. Most internal systems are well developed, but the lungs may still be immature.

Week 28
28 weeks after conception, baby’s eyes can open wide. Red blood cells are forming in bone marrow. Baby’s growth, in terms of her length, will soon slow down, but she or he will continue to gain weight. Baby weighs about 1.3 kilograms at this stage. Respiratory system is still quickly developing.

Week 29
Baby continues to gain weight. Due to the limited space in the womb (uterus), he’s now likely to have his legs drawn up to his chest, in the classic fetal position. Baby starts to look more like a newborn. His wrinkled skin is slowly becoming smoother.

Week 30
The layer of soft, downy hair that has covered baby’s skin for the past few months (lanugo) starts to fall off this week. Baby’s fingernails are fully formed and have reached the tips of fingers. Toenails grow to full length later than fingernails, but they are just visible at this stage.

Week 31
Baby now weighs about 1.9 kilograms, almost the same as a pineapple, and measures over 40 centimeters. The lungs are close to being fully developed at this point. Baby’s pupils can change size in response to a stimulus caused by light. His or her bones are hardening, however, the skull remains soft and flexible.

Week 32
At 32 week of development baby’s immune system develops. There is also produced more brown fat tissue.

Week 33
As baby takes up more space in the womb, the amount of amniotic fluid surrounding him is naturally decreasing so baby’s movement is slower. Baby now weighs nearly 2.4 kilograms and will gain about 30 grams a day over the next few weeks. Skin is becoming pink and smooth. His or her limbs have a chubby appearance.

Week 34
Baby’s toenails have reached the tips of his or her toes. Baby is preparing for delivery by laying in the uterus with head pointing downwards. Placenta works less efficiently.

Week 35
Now the baby is full-term and that means he or she is ready to be born. Baby continues to work his facial muscles by practicing pouts, frowns and grimaces.

Week 36
At 36 weeks after conception, the circumference of baby’s head and abdomen are about the same. Baby has mostly shed all of his or her lanugo. This week is also time for lens development.

Week 37
37 weeks after conception, the baby’s chest is becoming more prominent. For boys, the testes continue to descend into the scrotum. Fat is being added all over baby’s body to keep him or her warm after birth.

Week 38
Baby weights about 3.4 kilograms and measures about 50 centimeters. Baby’s development inside the uterus is complete. Usually at this week baby leaves uterus