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Within a week of conception, your fertilized egg known as a blastocyst will make its way to your uterus. The egg is about the size of a pen tip. In days the cells of the egg arrange themselves into groupings. The inner cell mass will become your baby, the outer cells will become the amniotic sac and placenta. The blastocyst then sheds its protective casing in a process called patching and barrows into the lush uterine wall.
Around week 5, your developing baby is the size of a sesame seed. The cells that once formed the blastocyst inner cell mass begin organizing and arranging, giving shape to the young embryos and forming primitive organs. Your baby’s brain and spinal cord are visible from his translucent skin. Right around this time, your baby circulatory system also forms and his heart begins to beat. Your baby looks more like a tiny tadpole than a human. He is drawing nutrients and oxygen through the newly formed placenta and umbilical cord.
By week 9, the embryonic tail is gone. Your baby is looking more human every day. With protruding limbs and fingers, undefined nose, mouth and eyes and tiny air lubes. Your new resident is about the size of a grain and weighs a fraction of an ounce. It is hard to believe how rapidly one cell evolves in such a short time into the unmistakable body of a baby.
At 10 weeks and barely the size of a kumquat, your baby is entering the fetal stage of development. His facial features are defined and his tooth buds are forming. Over the next week, his tissues and organs will rapidly grow and mature. The webbing is gone between his fingers and toes, and his nails and fingerprints are developing. Your baby can opening closes his fits and curl his toes. Thanks to his developing muscles and reflexes, he is now moving his limbs and kicking up a storm. If this is your first baby though, you likely won’t feel his flutters until 18 to 20 weeks. Through translucent skin, his vital organs are visible and functioning, including his growing brains, nervous system, intestines and liver, which is making red blood cells in place the disappearing yolk sac. The umbilical cord is working hard now, one vein delivers oxygen and nutrient rich blood to your baby. Two arteries then carry the blood away.
Around week 12 the kidneys have begun producing urine, which your baby will soon start excreting into the amniotic fluid. He will swallow the fluid and the process will begin again.
By week 14, your baby’s eyes and ears have moved into place, and he can squint, frown and grimace. You are now beginning the second trimester of pregnancy, which many women say is when they feel their best.
Between 15 and 20 weeks, your baby will more than double in size. As his body grows, his nervous system is rapidly maturing. His nerves are connecting his brain to the rest of his body. Traveling from the brain, through the brain stem and down the spine, and beginning to extend into his torso and limbs. Your baby skeleton is changing too, the soft cartilage is starting to harden into bone. This happens first in the arms and legs. Sensory development is picking up speed. Your baby’s brain is designating special areas for smell, taste, hearing, vision and touch. At this stage, your baby may be able to hear your heartbeat and voice. So, read aloud or sing a happy tune. Your baby may even be sucking his thumb nail.
At about 18 weeks, you will start to experience one of the most exciting parts of pregnancy, feeling your baby’s movement. His flexing arms and legs may feel like gentle flutters at first, they will become even stronger and more frequent in the weeks ahead.
At 20 weeks, your baby weighs a little more than ten ounces and measures about 10 inches from head to heel. About the length of a banana, this is a big milestone, the halfway point in your pregnancy.
Between 21 and 27 weeks, your baby’s sense of touch is developing, along with his unique fingerprints. When you rub or gently press on your belly, you may feel your baby squirm or kick in respond, he can feel his own face now as well as anything within his reach, including the umbilical cord. He will keep experimenting with and refining his sense of touch.
Around 23 weeks, your baby can swallow. Soon after, you may notice little hiccup. Do not worry, hiccup are normal part of development. And it is common for your baby to have them several times a day. His lungs are also forming millions of tiny branches called bronchus. Incredibly, your baby is already practicing breathing by inhaling and moving small amount of amniotic fluid fluid in and out of his lungs.
At 24 weeks, the baby is capable of surviving outside the uterus, with significant medical help.
At 27 weeks, your baby’s eye lids may open as a reflex, but he can not see yet. His lips are form, and he now have tiny taste bud on his tongue. He weights nearly 2 pounds, and is about 14 1/2 inches long.
Now, there is only one more trimester to go.
You and your baby are beginning your last trimester together. At 28 weeks, your baby weights just over 2 pounds. He will more than triple his weight between now and birth. And build a layer of fat that will help keep him warm outside the womb. Your baby’s skeleton is hardening. The bones in his head have not yet fused together, which will allow them to slightly overlap as his head passes through the snug space of the birth canal. Billions of neurons have developed and are firing in your baby’s brain. Creating essential connections that will help him learn in and out side the womb. Your baby can open his eyes now, and see the lights that filters through your womb. He can hear too, so go ahead and talk and sing to your baby. As your due date nears, your baby will shed small bits of vernix caseosa, the white “cheesy” substance that covers his entire body and protects his skin from the amniotic fluid he’s floating in. Your baby swallows some of these bits, along with other secretions, which pass through his digestive system to become his first bowel movement.
At 37 weeks, your baby’s skin is pink and soft. He’s looking less like a wrinkled alien and more like a baby. He’s gaining nearly one ounce a day and weighs about 6 pounds. He’s now considered full-term.
In preparation for birth, most babies will turn, so their head is facing downward. Your baby will stay that way, head resting in your pelvic area, as he gets ready to greet the world.