We all know how life begins: One sperm out of millions wins the race to meet the egg and burrows inside to fertilize it, uniting the mother's and father's genes.
But what happens next to create the precious and unique human being you meet on delivery day 40 weeks later? As you're suffering through morning sickness, heartburn, and growing pains, and preparing baby's nursery, your body is making a home for your child before birth -- and supporting it through the miraculous changes that take it from a ball of cells to a fully formed newborn. Here's a peek at how your baby develops during your pregnancy.
Once conception occurs, the fertilized egg divides rapidly as it floats down the fallopian tube to the uterus. Eight days later, this ball of about 200 cells attaches to the lining of the uterus and settles in for the next nine months.
Now an embryo, it folds into itself at five weeks so that it looks more like a tube than a ball. The ends tuck in -- one will be the head, the other, the beginning of baby's cute behind -- and between them the spinal cord is developing. Also forming are your unborn baby's lifelines to you: the placenta and umbilical cord. By six to eight weeks, the embryo looks more like a tadpole than like a baby -- it even has a tail that will disappear as the embryo grows.
The baby-to-be floats in the amniotic sac, a supple bag inside the uterus that fills with fluid over time to cushion and protect him. Though only half an inch long, the embryo has a beating heart, and its limbs, fingers, and toes are forming. The eyes also develop early from mere indentations on either side of the face to light-discerning organs.
By now, a complete set of miniature internal organs has formed and begun functioning, making the unborn baby a fetus. If you could peer into the womb, you would see its tiny organs through the almost transparent skin. The umbilical cord, which carries food to and waste from the fetus, is thick and strong.
Some of the intestines will grow within the cord for now because the fetus's stomach is too small to hold them all. (They'll move into the body next month.) Muscles and soft, rubbery bones are growing to support the organs. On the fetus's head, tiny buds for ears appear and eyelids form and close over the eyes.
By the middle of this month, the external genitals are distinctly formed, making the fetus recognizably a girl or a boy. Facial features are also becoming more distinguishable, teeth are growing underneath baby's gums, and the fingers and toes are becoming longer and more defined.
The baby's senses begin to awaken, too. Studies show that if a bright light is shone into the uterus, the fetus will shield his eyes. He's also beginning to hear what's going on inside and out of the womb: The sound of your voice, heartbeat, and growling stomach will become familiar and reassuring.
The fluttering sensation you feel in your abdomen this month means the fetus is moving -- and that's living proof that his muscles and limbs are getting stronger and his bones are hardening. He can kick, turn, curl his toes, frown, or purse his lips -- even hiccup -- all while floating around in about a pint of amniotic fluid, made up mostly of fetal urine. (Though that may sound yucky, it's not harmful to the fetus, who actually swallows it.)
Baby's sucking reflex has also developed. If his hand floats to his mouth, he can suck his thumb. At 5 inches and 4 ounces, the fetus is now big enough to fill the palm of your hand.
Now a tiny version of a newborn, the fetus is wrinkly, hairy, and covered with a cheese-like coating, called vernix, which protects his skin from constant exposure to amniotic fluid. You might feel him startle at loud noises because your unborn baby's hearing is near-perfect now.
His heartbeat is also strong enough for your doctor to detect clearly with a stethoscope, and his lungs are starting to prepare to breathe on their own. For now, however, the placenta exchanges oxygen in your blood with carbon dioxide in the fetus's. Baby's movements are more coordinated, too, and if you push at a jutting arm or leg, you may find that he pushes back!
By the end of the month, your baby is about 10 inches long and weighs 1 1/2 pounds.
Things are getting a bit cramped in there, so your unborn baby needs to stay in the fetal position. Even though there's not a lot of space, his kicks and pushes don't let up. By now you probably notice he has a regular, daily pattern of activity and rest.
This month the eyelids open for the first time (his pupils even grow larger or smaller in response to brightness), and the lanugo is beginning to disappear. Eyelashes and the hair on his head are growing, though some babies are born bald. Now and until birth the fetus is also fattening up, which makes his skin smoother and pinker.
The fetus is getting ready for his big coming-out party. He gains more than half his birth weight in the last seven weeks. The right and left parts of his brain begin working together, and the nervous system and the lungs are maturing. Plus, a boost to the fetus's immune system comes from your antibodies, which are passed through the placenta. Near the end of the month, most babies assume the position for delivery: head down, facing your back. Make way!
Originally published in American Baby magazine, November 2004.
All content here, including advice from doctors and other health professionals, should be considered as opinion only. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.