Week4Baby (called an embryo) consists of two layers of cells — the epiblast and the hypoblast — that will eventually develop into all of baby's organs and body parts. Two other structures that develop at this time are the amnion and the yolk sac. The amnion, filled with amniotic fluid, will surround and protect the growing embryo. The yolk sac will produce blood and help to nourish the embryo until the placenta takes over that role.

Week5The neural tube, which will eventually form into the spinal cord and brain, runs from the top to the bottom of the embryo. A bulge in the centre of the embryo will develop into baby's heart. At this time, the placenta develops. It is through the placenta and its finger like projections, called chorionic villi, that an embryo receives nourishment from its mother.

Week6Baby’s brain and nervous system are developing at a rapid pace. Optic vesicles, which later form the eyes, begin to develop

this week on the sides of the head, as do the passageways that will make up the inner ear. Baby's heart will begin to beat around this time, and it may even be detected on ultrasound examination. And the beginnings of the digestive and respiratory systems are forming, too. Small buds that will grow into baby's arms and legs also appear in this week. Baby only measures 0.08 to 0.2 inches (2 to 5 millimeters) from crown to rump.

Week7Baby is constantly adapting to life inside the uterus. The umbilical cord has formed. It will be baby’s connection to mother throughout the pregnancy, providing oxygen and nourishment for the baby and disposing of baby's wastes. In addition, baby's digestive tract and lungs continue to form. Baby's face is taking shape. The mouth, nostrils, ears, and eyes are some of the facial features that become more defined this week. The arm bud that developed just last week has a hand on the end of it.

Week8

The fingers and toes are just beginning to form this week, and the arms can even flex at the elbows and wrists. The eyes are becoming more obvious because they’ve begun to develop pigment in the retina. Also, the intestines are getting longer and there isn’t enough room for them in the baby’s abdomen, so they protrude into the umbilical cord until week 12.By now, the beginnings of the buds that will develop into baby's genitals have made their appearance.

Week9The tail at the bottom of baby's spinal cord has shrunk and almost disappeared by this week. In contrast, baby's head has been growing — it's quite large compared with the rest of the body and it curves onto the chest. By this week, baby measures about 0.6 to 0.7 inches (16 to 18 millimeters) from crown to rump and weighs around 0.1 ounces (3 grams). The tip of the nose has developed and can be seen in profile, and flaps of skin over the eyes have begun to shape into eyelids. The digestive system continues to develop. The anus is forming, and the intestines are growing longer. In

addition, internal reproductive features, such as testes and ovaries, start to form this week. Baby may make some first movements this week as muscles develop.

Week10All of baby's vital organs have been formed and are starting to work together. As external changes such as the separation of fingers and toes and the disappearance of the tail takes place, internal developments are taking place too. Tooth buds form inside the mouth, and if baby is a boy, his testes will begin producing the male hormone testosterone. Congenital abnormalities are unlikely to develop after week 10. This also marks the end of the embryonic period — in general, the embryo now has a distinctly human appearance and starting next week baby will officially be considered a fetus.

Week11From this week until week 20, baby will be growing rapidly — increasing in size from about 2 inches (5 cm) to about 8 inches (20 cm) from crown to rump. To accommodate all this growth, the blood vessels in the placenta are increasing in both size and

number to provide the baby with more nutrients. Facial development continues as the ears move toward their final position on the sides of the head. The baby's head accounts for about half of the body length. Baby's reproductive organs are developing rapidly, the external genitals of boys and girls appear somewhat similar until the end of week 11.

Week12Baby's brain continues to develop, and tiny fingernails and toenails start to form. Vocal cords are formed this week, which is the last of first trimester. Baby's kidneys are functioning. After swallowing amniotic fluid, baby will now be able to pass it out of the body as urine. And the intestines will make their way into the abdomen, since there is room for them now.

Week13At beginning of the second trimester of pregnancy, placenta has developed and is providing baby with oxygen, nutrients, and waste disposal. The placenta also produces the hormones progesterone and estrogen, which help to maintain the pregnancy. By

now, the baby's eyelids have fused together to protect the eyes as they develop.

Week14By this week, some fine hairs have developed on baby's face. This soft colourless hair is called lanugo, and it will eventually cover most of baby's body until it is shed just before delivery. By now, baby's genitals have fully developed, though they may still be difficult to detect on an ultrasound examination. In addition, baby starts to produce thyroid hormones because the thyroid gland has matured. Baby now weighs about 1.6 ounces (45 grams) and is about 3.5 inches (9 cm) long from crown to rump.

Week15Baby’s skin has been continuously developing, and it is so thin and translucent that we can see the blood vessels through it. Hair growth continues on the eyebrows and the head. Baby's ears are almost in position now, although they are still set a bit low on the head. Internally, Baby's skeletal system continues to develop. Muscle

development continues too, and baby is probably making lots of movements with her head, mouth, arms, wrists, hands, legs, and feet.

Week16Baby now weighs about 3.9 ounces (110 grams) and measures about 4.7 inches (12 cm) in length from crown to rump. Baby can hold her head erect, and the development of facial muscles allows for a variety of expressions, such as squinting and frowning.

Week17At about 5.1 inches (13 cm) from crown to rump and weighing 4.9 ounces (140 grams), baby is still very tiny. The placenta, which nourishes the fetus with nutrients and oxygen and removes wastes, is growing to accommodate baby. It now contains thousands of blood vessels that bring nutrients and oxygen from mother’s body to baby's developing body.

Week18

Ears move to their final position and they stand out from the head. The bones of the middle ear and the nerve endings from the brain are developing so that baby will hear sounds such as heartbeat and blood moving through the umbilical cord. She may even be startled by loud noises. Baby's eyes are also developing — they're now facing forward rather than to the sides, and the retinas may be able to detect the beam of a flashlight. Until now, Baby's bones had been developing but were still soft. This week, they begin to harden, or ossify. Some of the first bones to ossify are those in the clavicles and the legs.

Week19Baby is now covered with a white, waxy substance called vernix caseosa, which helps prevent delicate skin from becoming chapped or scratched. Premature babies may be covered in this cheesy coating at delivery. Baby is still tiny, but this week brings the development of brown fat, which will help keep the baby warm after birth. During the last trimester, baby will add more layers of fat for warmth and protection.

Week20Baby now weighs about 11 ounces (312 grams) and measures about 6.3 inches (16 cm) from crown to rump. The baby is taking up increasing room in the uterus, and continued growth will put pressure on mother’s lungs, stomach, bladder, and kidneys. Under the vernix caseosa, baby's skin is thickening and developing layers. Hair and nail growth continue.

Week21The amniotic fluid that has cushioned and supported the baby in the uterus now serves another purpose. The intestines have developed enough that small amounts of sugars can be absorbed from the fluid that is swallowed and passed through the digestive system to the large bowel. Almost all of the baby's nourishment, however, still comes from mother through the placenta. Until now baby's liver and spleen have been responsible for the production of blood cells. But now the bone marrow spaces are developed enough to contribute to blood cell formation as well, and bone marrow will become the major site of blood cell

production in the third trimester and after birth. (The spleen will stop producing blood cells by week 30, and the liver will stop a few weeks before birth.)

Week22Taste buds have started to form on the tongue, and the brain and nerve endings are formed enough so that the fetus can feel touch. Baby may experiment with this newfound sense of touch by stroking her face or sucking on a thumb, as well as feeling other body parts and seeing how they move. Baby's reproductive system is continuing to develop, too. In boys, the testes have begun to descend from the abdomen, and in girls, the uterus and ovaries are in place and the vagina is developed.

Week23Even though fat is beginning to accumulate on baby's body, the skin still hangs loosely, giving baby a wrinkled appearance. Baby's daily workout routine includes moving the muscles in the fingers, toes, arms, and legs regularly. By now baby weighs a little more

than 1 pound (454 grams). If preterm labor and delivery were to occur this week, a baby could survive with expert medical care, but might have mild to severe disabilities.

Week24Baby is still receiving oxygen through the placenta. But once birth occurs, her lungs will start taking in oxygen on their own. In preparation for that, baby's lungs are developing the ability to produce surfactant. Surfactant is a substance that keeps the air sacs in our lungs from collapsing and sticking together when we exhale, allowing us to breathe properly. Because the inner ear — which controls balance in the body — is now completely developed, baby may be able to tell when she is upside down or right side up while floating and making movements in the amniotic fluid.

Week25Baby has resting and alert periods. Fetal activity is more readily when mother is more sedentary. Baby's hearing has continued to develop, too —she may now be able to hear voice.

Week26Although baby's eyes have been sealed shut for the last few months, they will soon open and begin to blink. Depending on ethnicity, some babies will be born with blue or gray-blue eyes (which may change color in the first year of life) and some will be born with brown or dark eyes. Eyelashes are growing in, as is more hair on the head. Baby, weighing a little less than 2 pounds (907 grams), still looks wrinkly but will continue to gain weight steadily over the next 14 weeks until birth.

Week27By this first week of the third trimester, baby looks similar to what she will look like at birth, except thinner and smaller. The lungs, liver, and immune system still need to fully mature, but if born now, baby would have a very good chance of surviving. As hearing continues to develop, baby may start to recognize voice. Sounds may be muffled, though, because the ears are still covered with vernix, the thick waxy coating

that protects the skin from becoming chapped by the amniotic fluid.

Week28Baby now weighs about 2 pounds, 2 ounces (1,000 grams) and measures about 10 inches (25 cm) from crown to rump. Babies who are in the breech position may need to be delivered by cesarean section. Baby still has 2 months to change position. Most babies will switch positions on their own. The folds and grooves of baby's brain continue to develop and expand. In addition, baby continues to add layers of fat and has continued hair growth.

Week29Baby continues to be active, and starts first few flutters of movement like hard jabs and punches. Baby should move at least 10 times in two hours. If baby moves less, talk to health care provider.

Week30Now weighing about 3 pounds (1,400 grams) and measuring about 10.8 inches (27 cm) from crown to rump, baby continues to gain weight and layers of fat. This fat makes the baby look less wrinkly and will

help provide warmth after birth. In preparation for respiration after birth, baby will mimic breathing movements by repeatedly moving the diaphragm. Baby can even get the hiccups, which can be felt as rhythmic twitches in the uterus.

Week31

By now baby is urinating approximately several cups of urine a day into the amniotic fluid. She is also swallowing amniotic fluid, which is completely replaced several times a day. Excess fluid in the amniotic sac (known as polyhydramnios) may mean that the baby isn't swallowing normally or that there is a gastrointestinal obstruction. Inadequate fluid in the amniotic sac (oligohydramnios) may mean that the baby isn't urinating properly and could indicate a problem with the kidneys or urinary tract.

Week32

Eyelashes, eyebrows, and the hair on baby's head are evident. The lanugo hair that has covered your baby since the beginning of the second trimester is falling off, although some may remain on the shoulders and

back at birth. At about 4 pounds (1,800 grams) and 11.4 inches (29 cm) from crown to rump, baby would have an excellent chance of survival outside the womb if delivered now.

Week33

In these last few weeks before delivery, the billions of developed neurons in baby's brain are helping her to learn about the in-utero environment — baby can listen, feel, and even see somewhat. Baby’s eyes can detect light and the pupils can constrict and dilate in response to light. Like a newborn, baby sleeps much of the time and even experiences the rapid eye movement (REM) stage. Baby's lungs are almost completely matured. Fat will continue to be deposited on baby's body for protection and warmth. Babies gain a good deal of their weight in the final few weeks before birth.

Week34

Maternal calcium intake is extremely important during pregnancy because the baby will draw calcium from the mother to make and harden bone. If a pregnant woman doesn't get enough calcium during

pregnancy, it can affect her own bones because the developing fetus will take minerals from the mother's skeletal structure as needed. The vernix coating on the baby's skin is becoming thicker, whereas lanugo hair is almost completely gone. By now most babies will be in position for delivery. Babies born at 34 weeks usually have fairly well-developed lungs, and their average size of 5 pounds (2,250 grams) and 12.6 inches (32 cm) from crown to rump allows them to survive outside the womb without extensive medical intervention.

Week35

Baby already weighs about 5 pounds, 5 ounces (2,400 grams), but this week begins baby's most rapid period of weight gain — about 8 to 12 ounces (226 to 340 grams) each week. Fat is being deposited all over baby's body, especially around the shoulders. Because of this increasing size baby is now cramped and restricted inside the uterus — so fetal movements may decrease, but they may be stronger and more forceful. If baby is in a headfirst position, her head will rest on mother’s pubic bone in preparation for labor.

Week36

The wrinkly, tiny fetus seen on earlier ultrasounds has given way to an almost plump baby. There is fat on baby’s cheeks, and powerful sucking muscles also contribute to baby's full face. Baby now weighs a little under 6 pounds (2,721 grams).The bones that make up baby’s skull can move relative to one another and overlap each other while baby’s head is inside mother’s pelvis. This phenomenon is called molding, and it helps the baby pass through the birth canal.

Week37

By now, baby has developed enough coordination to grasp with the fingers. If shown a bright light, baby may turn toward it in the uterus. She continues to gain weight — developing fat at the rate of half an ounce (14 grams) a day. In general, boys weigh more than girls at birth.

Week38

Baby weighs about 6 pounds, 6 ounces (2,900 grams) by now and measures about

13.4 inches (34 cm) in length from crown to rump. Fat is still accumulating, although growth is slower now. Weight gain has decreased or ceased. Since baby has had the muscles to suck and swallow amniotic fluid, waste material has been accumulating in her intestines. Cells shed from the intestines, dead skin cells, and lanugo hair are some of the waste products that contribute to meconium, a greenish-black substance that constitutes baby's first bowel movement. If it is a boy, his testicles have descended into the scrotum. If it is a girl, the labia are now completely developed.

Week39

Umbilical cords, which carry nutrients from the placenta to the baby, vary in size but average about 22 inches (55 cm) long and half an inch (1-2 cm) thick. Sometimes the umbilical cord can become wrapped around a baby’s neck. Generally, this doesn’t cause problems, although a cesarean delivery could be required if it causes pressure on the umbilical cord during labor or delivery. A true knot in the umbilical cord is much less common, occurring in only about 1% of pregnancies. Most of the vernix that covered baby's skin has disappeared, as has the

lanugo. Mother’s body has been supplying the baby with antibodies through the placenta that will help the baby's immune system fight infection for the first 6-12 months of life.

Week40

After many weeks of anticipation and preparation, baby is here. Or maybe not — only 5% of women deliver on their estimated due dates, and many first-time mothers find themselves waiting up to 2 weeks after their due date for their baby to arrive. A baby born at 40 weeks weighs, on average, about 7 pounds, 4 ounces (3,300 grams) and measures about 20 inches (51 cm). Newborns often have heads temporarily misshapen from the birth canal and may be covered with vernix and blood. Baby's skin may have skin discolorations, dry patches, and rashes — these many variations are completely normal.