Within a week of conception, the fertilized egg, known as a blastocyst, will make its way to the uterus/womb. The egg is about the size of a grain of sugar. In days, the cells in the egg arrange themselves into groupings. The inner cell mass will become the baby. The outer cells will become the amniotic sac and placenta. The blastocyst then sheds its protective casing in a process called hatching, and burrows into the lush uterus wall. Around week 5, the developing baby is the size of a sesame seed. The cells that once formed the blastocyst's inner cell mass begin organizing and arranging, giving shape to the young embryo and forming primitive organs. The baby's brain and spinal cord are visible through his translucent skin. Right around this time, the baby's circulatory system also forms and his heart begins to beat. The baby looks more like a tiny tadpole than a human. He's drawing nutrients and oxygen through the newly formed placenta and umbilical cord. By week 9, the embryonic tail is gone. The baby's looking more human every day, with protruding limbs and fingers, a defined nose, mouth, and eyes, and tiny earlobes. The baby is about the size of a grape and weighs a fraction of an ounce. It's hard to believe how rapidly one cell evolves in such a short time into the unmistakable body of a baby!
At ten weeks and barely the size of a kumquat, the baby is entering the fetal stage of development. His facial features are defined, and his tooth buds are forming. Over the next weeks, 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. The baby can open and close his fists and curl his toes. Thanks to his developing muscles and reflexes, he's now moving his limbs and kicking up a storm. Through translucent skin, his vital organs are visible and functioning, including his growing brain, nervous system, intestines, and liver, which is making red blood cells in place of the disappearing yolk sac. The umbilical cord is working hard now. One vein delivers oxygen and nutrient-rich blood to the baby. Two arteries then carry the blood away. Around week 12, the kidneys have begun producing urine. By week 14, the baby's eyes and ears have moved into place, and he can squint, frown, and grimace. The second trimester of pregnancy has begun, which many women say is when they feel their best.
Between 15 and 20 weeks, the 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 brainstem and down the spine, and beginning to extend into his torso and limbs. The baby's 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. The baby's brain is designating special areas for smell, taste, hearing, vision, and touch. At this stage, the baby may be able to hear his mom’s heartbeat and voice. The baby may even be sucking his thumb now. At about 18 weeks, the mother will start to experience one of the most exciting parts of pregnancy: feeling her baby's movements. His flexing arms and legs may feel like gentle flutters at first. They'll become even stronger and more frequent in the weeks ahead. At 20 weeks, the baby weighs a little more than 10 ounces and measures about 10 inches from head to heel (about the length of a banana). This week is a big milestone – the halfway point in the pregnancy.
Between 21 and 27 weeks, the baby's sense of touch is developing, along with his unique fingerprints. He can feel his own face now, as well as anything within his reach, including the umbilical cord. He'll keep experimenting with and refining his sense of touch. Around 23 weeks, the baby can swallow. Soon after, the mother may notice small hiccups. His lungs are also forming millions of tiny branches, called bronchioles. Incredibly, the baby is already practicing breathing by inhaling and moving small amounts of amniotic fluid in and out of his lungs. At 24 weeks, a baby is capable of surviving outside the uterus with significant medical help. At 27 weeks, the baby's eyelids may open as a reflex, but he can't see yet His lips are formed, and he now has tiny taste buds on his tongue. He weighs nearly 2 pounds and is about 14 and a half inches long. At 28 weeks, the baby weighs just over 2 pounds. He'll more than triple his weight between now and birth, and build a layer of fat that will help keep him warm outside the womb.The 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 the baby's brain, creating essential connections that will help him learn in and outside the womb.The baby can open his eyes now and see the light that filters through the womb.At 37 weeks, the baby's skin is pink and soft. He's looking less wrinkled 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. The baby will stay that way as he gets ready to greet the world!
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