Blowing out the birthday candles means you've grown another year older. But did you know? In fact, your real "birthday" is actually a certain day nine months before you were born. On that day, the father's sperm and the mother's egg merged to form a "zygote". This zygote, which is even smaller than a full stop, is the true starting point of the journey of life. Before facing the world independently, you stayed in your mother's body for about 266 days. Your mother specially built a "palace" for your growth - the uterus. During this period, you grew from a zygote the size of a needle tip to a fetus weighing about 3.5 kilograms and containing 10^12 cells. It's not an exaggeration to describe your changes in the uterus as earth-shattering.
Let's take a "time machine" to revisit your time in your mother's uterus. Before the zygote was formed, the father generously sent hundreds of millions of sperm into the mother's body, while the mother was already prepared: one of her eggs was waiting in the fallopian tube. When the vast army of sperm arrived at the fallopian tube, the leading sperm was invited by the egg and entered it, while the other sperm lost the opportunity to combine with the egg. In this way, after the egg accepted the sperm, it became the first you - a zygote.
As soon as the zygote was formed, it began to self-replicate non-stop. One became two, two became four... While the number of cells was doubling, it was gradually moving from the fallopian tube to the uterus. Four days later, the original zygote had become a "meat ball" with a hollow center, consisting of hundreds of cells. Scientists gave it a vivid name - the embryo sac. In the middle of the embryo sac was a large cavity, called the embryo cavity. Surrounding the embryo sac was a layer of cells, called the trophoblast. In addition, on one side of the embryo sac, there was a cluster of cells connected to the trophoblast, called the inner cell mass. When the embryo sac was just formed, it couldn't find a place to settle and was like a wandering child in the fallopian tube. About two days later, it began to gradually approach the uterine lining and finally fell into the fertile soil of the uterine lining like a seed, which is called implantation. It took about a week for the embryo sac to completely implant. By this time, the embryo sac had already settled firmly in the uterine lining and could better receive the nutrients and protection provided by the mother. At this time, in addition to the increase in the number of cells through division, the types of cells also diversified. The original meat-ball-like embryo sac began to become less round. It grew inward, forming new septae, which were called the amniotic sac on the inside and the chorion on the outside. In the chorion, the blood vessels of the mother and the embryo were close but not connected, ensuring that the mother and the embryo could exchange nutrients and waste, oxygen and carbon dioxide, while their blood was independent and did not mix. At this time, the "embryo sac" was officially renamed the "embryo".
When the journey of life enters the fifth week, the embryo develops into three layers, and all the organs of your body will gradually differentiate from these three embryonic layers. The ectoderm will form the nervous system and the spine, the mesoderm will form the heart and the circulatory system, and the endoderm will form the lungs, intestines, stomach, digestive tract, thyroid gland, liver and pancreas. At this time, although you are only the size of a sesame seed, you already have the rudimentary brain, spinal cord and heart. A umbilical cord connecting you and your mother's body is emerging, which can better assist the development of the embryo. Now, touch your navel, and it is the "relic" of the umbilical cord connecting you and your mother.
By the end of the eighth week, the embryos of various organs have taken shape. At this time, your head is very large, accounting for almost half of your entire body. Although you have eyes, ears, nose and mouth, your nose is flat, and your two eyes are also very far apart. Although this appearance is not very beautiful, it already looks like a human being.You're starting to look more like a "human" now, and the once-briefly present tail has disappeared. Around the 9th week, you've grown to the size of a walnut, and your head and body have become distinguishable. Your kidneys, liver, brain, and lungs begin to function independently, while your fingers and toes start to differentiate and your external genitalia form. At this point, since your appearance clearly shows human features, you can no longer be ambiguously called an embryo. Thus, from the 9th week onward, you're referred to as a "fetus," which will be your final name in the womb.
By the 15th week, you weigh about as much as an egg. Although your eyelids are still fused together, your retina can already detect light. Your appearance is already strikingly similar to your future self in a newborn photo. Your bones and muscles continue to strengthen, and your increasingly vigorous limb movements give expectant mothers a clearer sense of life movement. By the end of the sixth month, you've grown to 20 centimeters in length and weigh nearly 700 grams. With only three months left in the womb, grooves and ridges appear on the surface of your brain, your lungs develop fully, your eyelids can open, and your height has doubled compared to three months earlier, with your weight reaching over 3 kilograms. You can now open and close your eyes, and subcutaneous fat continues to accumulate as your body grows larger. Your limbs become rounder, your skin gradually turns pink, and you often kick vigorously against your mother's belly—you're eager to come out and explore the world!
Finally, when the moment of delivery arrives, medical staff cut the umbilical cord connecting you to your mother, and you cry out into the world, ready to face this amazing place independently.