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First Trimester - Fertilization
The moment that a single sperm has broken through the tough outer membrane of an egg and fertilized it. After a few days the fertilized egg make its way to the uterus and starts burrowing into the lining. A baby is in the making! -
Week 3
The baby is called a blastocyst, consisting of several hundred cells that are multiplying rapidly. The amniotic fluid is starting to collect around the cells in the cavity that becomes the amniotic sac. The blastocyst is receiving oxygen and nutrients through the blood vessels in the uterine wall. The placenta won't be developed enough to take over this task until the end of next week. -
Week 4
All of the baby's organs start to develop and a few of them will start to function. At this time she'll be more vulnerable to anything that might interrupt with her development. The baby is the size of a poppy seed, consisting of two layers: the epiblast and the hypoblast, which forms all of her organs and body parts that will develop. -
Week 5
The embryo is growing at a rapid pace. The baby is the size of a sesame seed, and looks like a tiny tadpole. She's made up of three layers — the ectoderm, the mesoderm, and the endoderm, which will later form all of her organs and tissues. -
Week 6
This week's major developments: The nose, mouth, and ears. The head is oversize and dark spots where the baby's eyes and nairs are starting to form. The legs and arms are protruding buds. The heart rate is about 100 to 160 beats per minute and blood is beginning to flow through the body. The intestines are developing, and buds of tissue starting the lungs has appeared. The pituitary gland, brain, muscles, and bones are forming. The baby is the size of lentil. -
Week 7
The hands and feet are turning into arms and legs, they look like little paddles. The baby is still has a small tail, which is an extension of her tailbone. The tail will disappear within a few weeks. The baby has doubled in size and is about the size of a blueberry. -
Week 8
The hands and feet are developing webbed fingers and toes, eyelids almost cover her eyes, breathing tubes extend from throat to the branches of developing lungs, and the "tail" is just about gone. In the brain, nerve cells are branching out to connect with one another, forming primitive neural pathways. Baby is the size of a kidney bean. -
Week 9
The baby's heart has dividing into four chambers (atria/ventricles), and the valves form, and tiny teeth form. The embryonic "tail" is gone. The baby's organs, muscles, and nerves are kicking into gear. Eyes are fully formed, but her eyelids are fused shut. She has earlobes, and her mouth, nose, and nostrils are more distinct. The placenta is developed enough to take over most of the critical job of producing hormones. Baby is the size of a grape. -
Week 10
The baby weighs less than a quarter of an ounce, the baby has completed the most critical part of its development. This is the beginning of the fetal period, a time when the tissues and organs in his body rapidly grow and mature. -
Week 11
The baby is just over 1 1/2 inches long and about the size of a fig, and now fully formed. Tiny tooth buds begin to appear under her gums, and some of her bones are beginning to harden. She's already making tiny movements. She may begin hiccuping now that her diaphragm is forming. -
Week 12
The baby's developments this week: reflexes. The baby's fingers will soon begin to open and close, her toes will curl, her eye muscles will clench, and her mouth will make sucking movements. Her intestines, which have grown so fast that they protrude into the umbilical cord, will start to move into her abdominal cavity about now, and her kidneys will begin excreting urine into her bladder. -
Week 13
The fingertips have fingerprints, her veins and organs are clearly visible through her still-thin skin, and her body is starting to catch up with her head. If you're having a girl, she now has more than 2 million eggs in her ovaries. The baby is about the size of a pea pod, almost 3 inches long and weighs nearly an ounce. -
Second Trimester - Week 14
Due to brain impulses. the baby can now squint, frown, grimace, pee, and possibly suck her thumb. The kidneys are producing urine. The baby is about the size of a lemon, she measures 3 1/2 inches and she weighs 1 1/2 ounces. She's developing an ultra-fine, downy covering of hair, called lanugo, all over her body. The baby's liver starts making bile, and her spleen starts helping in the production of red blood cells. -
Week 15
The baby is moving amniotic fluid through her nose and upper respiratory tract, which helps the primitive air sacs in her lungs begin to develop. Her legs are growing longer than her arms now, and she can move all of her joints and limbs. Although her eyelids are still fused shut, she can sense light. The baby is about the size of an apple, now measures about 4 inches long and weighs about 2 1/2 ounces -
Week 16
The baby's legs are much more developed, her head is more erect than it has been, and her eyes have moved closer to the front of her head. Her ears are close to their final position. The patterning of his scalp has begun. She's even started growing toenails and her heart is now pumping about 25 quarts of blood each day. The baby is about the size of an avocado, 4 1/2 inches long and about 3 1/2 ounces. -
Week 17
The baby's skeleton is changing from soft cartilage to bone, and the umbilical cord, her lifeline to the placenta, is growing stronger and thicker. She can move her joints, and her sweat glands are starting to develop. The baby weighs 5 ounces now and she's around 5 inches long. -
Week 18
Her blood vessels are visible through her thin skin, and her ears are now in their final position, and they stand out from her head. Myelin (a protective covering) begins to form around her nerves, a process that will continue her first year of life after she's born. Her uterus and fallopian tubes are formed and in place. If you're having a boy, his genitals are noticeable now, but he may hide them from you during an ultrasound. -
Week 19
Her brain is designating areas for smell, taste, hearing, vision, and touch, so read aloud, talk to her, or sing to her. Her arms and legs are in the right proportions to each other and the rest of her body now. The baby's kidneys continue to make urine and the hair on her scalp is sprouting. A waxy protective coating called the vernix caseosa is forming on her skin to prevent it from pickling in the amniotic fluid. The baby is about the size of an heirloom tomato. -
Week 20
She's producing meconium, a black, sticky by-product of digestion, you'll see it in her first soiled diaper (some babies pass meconium in the womb or during delivery). The baby weighs about 10 1/2 ounces now. She's also around 6 1/2 inches long from head to bottom and about 10 inches from head to heel, about the size of a banana. -
Week 21
You may notice a pattern in activity as you get to know her better. In other developments, your baby's eyebrows and lids are present now, and if you're having a girl, her vagina has begun to form as well. -
Week 22
Her lips, eyelids, and eyebrows are becoming more distinct, and eyes has formed, but her irises (the part of your eye that has color) still lack pigment. Fine hair (lanugo) that covers her body and the deep wrinkles on her skin, which she'll sport until she adds a padding of fat to fill them in. Inside her belly, her pancreas, essential for the production of some important hormones, is developing. -
Week 23
Blood vessels in her lungs are developing to prepare for breathing, and the sounds that your baby's increasingly keen ears pick up are preparing her for entry into the outside world. Her sense of movement well developed by now, your baby can feel you dance. The baby is now about the size of a large mango. She's more than 11 inches long and weighs just over a pound. -
Week 24
Her body is filling out proportionally and she'll soon start to plump up. Her brain is also growing quickly now, and her taste buds are continuing to develop. Her lungs are developing "branches" of the respiratory "tree" as well as cells that produce surfactant, a substance that will help her air sacs inflate once she hits the outside world. Her skin is still thin and translucent, but that will start to change soon. -
Week 25
She's beginning to exchange her long, lean look for some baby fat. As she does, her wrinkled skin will begin to smooth out and she'll start to look more and more like a newborn. She's also growing more hair and if you could see it, you'd now be able to discern its color and texture. -
Week 26
The network of nerves in your baby's ears is better developed and more sensitive than before. She may now be able to hear both your voice and your partner's as you chat with each other. She's inhaling and exhaling small amounts of amniotic fluid, which is essential for the development of her lungs. If you're having a boy, his testicles should soon begin to descend into his scrotum -
Week 27
The baby has developed, sleeping and waking patterns at regular intervals, and also opening and closing her eyes. While her lungs are still immature, they would be capable of functioning, with a lot of medical help if she were to be born now. She is about the size of a head of cauliflower, weighs almost 2 pounds and is about 14 1/2 inches long with her legs extended. -
Week 28
The baby's eyesight is developing, she may be able to see the light that filters in through the womb. She's also is developing billions of neurons in her brain and adding more body fat in preparation for life in the outside world. She eye lashes and can blink her eyes. -
Week 29
Her muscles and lungs are continuing to mature, and her head is growing bigger to make room for her developing brain. Her bones are soaking up lots of calcium. This trimester, about 250 milligrams of calcium are deposited in her hardening skeleton each day. -
Week 30
Her eyesight continues to develop, though it's not very keen; even after she's born, she'll keep her eyes closed for a good part of the day. When she does open them, she'll respond to changes in light but will have 20/400 vision. -
Week 31
She can turn her head from side to side, and her move her arms, legs, and body has begun to plump as needed fat accumulates underneath her skin. She's probably moving a lot, too, so you may have trouble sleeping because your baby's kicks and somersaults keep you up. -
Week 32
You're gaining about a pound a week and roughly half of that goes right to the baby. She now has toenails, fingernails, and real hair. Her skin is becoming soft and smooth as she plumps up in preparation for birth. -
Third Trimester - Week 33
The bones in her skull aren't fused together, which allows them to move and slightly overlap, thus making it easier for her to fit through the birth canal. She's rapidly losing that wrinkled and her skeleton is hardening. This bay is about the size of a pineapple. She weighs a little over 4 pounds and about 17-inches. -
Week 34
Her fat layers, which will help regulate her body temperature once she's born, are filling her out, making her rounder. Her skin is also smoother than ever. Her central nervous system is maturing and her lungs are continuing to mature as well. -
Week 35
She isn't likely to be doing somersaults anymore, but the number of times she kicks should remain about the same. Her kidneys are fully developed now, and her liver can process some waste products. Most of her basic physical development is now complete. The baby doesn't have much room to maneuver now that she's about the size of a honeydew melon, and over 18 inches long and weighs around 5 1/4 pounds. -
Week 36
She's shedding most of the downy covering of hair that covered her body as well as the vernix caseosa, the waxy substance that covered and protected her skin during her nine-month amniotic bath. The baby swallows both of these substances, along with other secretions, resulting in a blackish mixture, called meconium, will form the contents of her first bowel movement. -
Week 37
The next two weeks the baby's brain and lungs will become fully developed. -
Week 38
She has a firm grasp, her organs have matured and are ready for life outside the womb and she has really plumped up. She weighs about 6.8 pounds and she is over 19 1/2 inches long. -
Week 39
She continues to build a layer of fat to help control her body temperature after birth, but it's likely she already measures about 20 inches and weighs a bit over 7 pounds, about the size of a mini-watermelon. -
Week 40/41
It's time!!
