Pregnancy is one of the most amazing experiences you will ever have. As your body changes week by week, it's sometimes hard to imagine how your baby is changing week by week, too. This guide is specially written to give you a little glimpse into the weekly progress of your baby's development.
FIRST TRIMESTER
Week 1 – This week begins with the first day of your period, so you are not pregnant yet. It is counted as part of your 40 week pregnancy because it signals the start of the gestational cycle.
Week 2 – During this week your body is forming a new uterine lining, tissue that is extremely rich in blood and perfect for nourishing a fertilized egg. As part of this process, one of your ovaries is about to release a ripe egg into a fallopian tube.
Week 3 – Your egg meets up with sperm during this week, and when a single sperm succeeds in entering the ovum conception occurs. Your pregnancy adventure is now officially underway!
Week 4 – The fertilized egg, now called a blastocyst, moves from the fallopian tube into the uterus, where it attaches itself to the uterine lining. This is called implantation.
Week 5 – The blastocyst has turned into an embryo, with three distinct layers. The top layer, called the ectoderm, is where the nervous system will develop. The middle layer, called the mesoderm, is where the circulatory system, reproductive organs, muscles, bones, and kidneys will develop. The inner layer, called the endoderm, is where the bladder, liver, pancreas, and intestines will develop.
Week 6 – The embryo is about 1/17 of an inch now, and the heart has begun to beat. The ears, eyes, and mouth are beginning to develop, as are the arm and leg buds. The umbilical cord, literally the lifeline between you and the embryo, also forms during this week.
Week 7 – Very rapid growth is continuing this week; the embryo is about 1/3 of an inch. How small is this? About the same as a grain of rice! The legs and arms are developing, but toes and fingers are yet to come. The brain, nostrils, eyes, pancreas, intestines, and bronchi are all developing, too.
Week 8 – The fingers and elbows are starting to appear, as are the toes and feet. Ears and eyes are developing, and teeth are beginning to form. Inside the umbilical cord, intestines are forming.
Week 9 – This week sees the beginnings of bones, cartilage, and the tongue. The intestines move into the abdomen because the body has grown enough to make room for them. Thumb and fingers can be seen, but they still have a thin webbing of skin connecting them to each other.
Week 10 – The baby's joints are now mostly in place, including the ankle, knee, wrist, shoulder, and elbow. This allows the baby to move around, although it is still too small for you to feel this movement.
Week 11 – The baby is now called a fetus, and the most critical parts of its formation and development has ended. Starting the week at about one inch long and ending it at about two inches long, you can imagine how fast growth is during this time. The iris of the eye is starting to form, and the eyelids are fused shut. For the first time, the placenta begins functioning so blood can circulate between the uterus and the baby.
Week 12 – Your fetus now has nearly all of the body structures and internal organs completely formed. Between now and deliver these parts of the body will continue to grow. Fingernails and hair start to emerge, and the kidneys start to function. External genitals start to form, but they are still too difficult to see the sex for sure via ultrasound.
SECOND TRIMESTER
Week 13 – Now the face is starting to look more like, well, a face! The vocal cords are developing, too. In the abdomen, the pancreas is starting to make insulin and the liver is starting to excrete bile. The external genitals are fully formed, but still not visible via ultra sound. You'll have to wait until about Week 16 for that possibility.
Week 14 – Your baby now weighs about two ounces and is about 3 ½ inches long. The lungs begin their first attempts at inhaling and exhaling, essentially “practicing” for the real world. This will continue throughout the remainder of your pregnancy. Other areas of development include the hands, eyes, ears, and neck. As the neck gets longer the chin lifts up and stops resting on the chest. Your doctor may or may not be able to detect a heartbeat using a Doppler, but it's still early so don't panic if it takes another couple of weeks for this to happen.
Week 15 – Some babies can be seen sucking their thumbs at this point (via ultrasound, of course), and a very fine hair called lanugo is covering the body. The skin at this stage is still extremely thin, even transparent enough to see blood vessels underneath.
Week 16 – The baby's bones are becoming harder, which sometimes means you can begin feeling movements inside your uterus. If this is your first pregnancy it may be a couple more weeks before you can feel the baby moving. There's lots of room to move around inside, so the baby takes advantage of all that space. Toenails and fingernails are growing by now, and the legs have grown longer than the arms.
Week 17 – The baby is growing rapidly now. He or she is about the same size as the placenta, and a layer of fat is beginning to appear under the skin. Reflexes such as blinking, swallowing, and sucking are occurring, too.
Week 18 – Your baby now weighs about eight ounces or so, and has definitely taken on a human shape and appearance. The bowel is beginning to accumulate meconium, which will become the first bowel movement after the baby is born. Sometimes the baby will get the hiccups, which you might notice as a jerky, consistent movement.
Week 19 – The placenta is providing your baby with nourishment, and it is continuing to grow in order to keep up with the baby's needs. The skin becomes covered with a protective substance called vernix; it is cheesy and white in appearance, and prevents skin damage from being in contact with fluid for so many months at a time.
Week 20 – Congratulations! You're half way through your pregnancy and the baby is continuing to grow. He or she has regular sleep and wake cycles, roughly equivalent to those of a newborn baby.
Week 21 – Growth starts to slow down about now; the baby will continue growing, of course, just not at the extremely rapid rate it did up to this point. The heart is getting stronger, as are the legs. You'll notice more and more firm bumps as the legs get longer and stronger.
Week 22 – The baby's body is nearly fully developed now, but certainly not ready to be born. He or she is still very thin and weighs just under a pound.
Week 23 – The bones in the baby's middle ear are becoming harder and the body continues to grow. He or she is probably about a pound or so. Babies born at this stage do have a chance at survival, but it is very slim in most cases.
Week 24 – The body is filling out, which means your baby could gain as much as six or eight ounces in just this timeframe. Most of that weight comes from growth of the internal organs, body tissues, bones, and muscles.
Week 25 – The spinal structures begin to form in earnest; when fully formed there will be 1,000 ligaments, 33 rings, and 150 joints. The lungs are changing, too, with more blood vessels continuing to form.
Week 26 – Your baby is about nine inches long now, and may weigh as much as two pounds. The lungs are forming air sacs now, and secreting a substance called surfactant. Surfactant coats the inside of the air sacs and allows them to expand and contract during breathing. Brain activity also increases during this week, with development in the ability to see and to hear.
Week 27 - The lungs and brain are growing quickly now. The retinas of the eyes are also forming, and the eyelids start to open and close. Your baby can grow as much as ½ inch during this week of development.
THIRD TRIMESTER
Week 28 – Congratulations! You're now starting the third trimester, and the baby is developing in many exciting ways. Eyelashes, eyebrows, and hair on the head are all present, and the eyes are now fully formed. The baby's muscle tone continues to strengthen, and the body is beginning to take on a plumper, more round shape. He or she can hear your voice very well, so talk to your baby whenever you like.
Week 29 – The baby's body and head are now in the proper proportion to each other, and the eyes can even move around in the eye sockets. The brain is continuing to develop; it now has the beginnings of the ability to regulate body temperature and breathing. The baby is also becoming responsive to sound, light, smell, and taste.
Week 30 – The baby's bone marrow is now producing red blood cells. Toenails and fingernails continue to grow, and the lanugo is starting to disappear. Your baby is approaching three pounds or so, and will continue to gain weight rapidly from this point forward.
Week 31 – The brain is in a phase of extremely rapid development now. All of the major organs are developed, except for the lungs.
Week 32 – Your baby is still growing, and may be up to four pounds or so by now. All five of his or her senses are fully functional, and hair on the head is growing rapidly as well.
Week 33 – Inside the amniotic sac, there is now a great deal of amniotic fluid. The baby's head grows a lot this week, due to the rapid growth in brain development. The skin begins to look more pink than red, thanks to the growing accumulation of fat underneath.
Week 34 – The baby is starting to do many of the same things as a newborn does, including opening and closing the eyes when awake and asleep. Another important development is the appearance of immunities necessary to help fight any mild infections.
Week 35 – Your baby is probably around five pounds or so by now. The arms and legs are becoming much plumper, thanks to the continuing development of fat deposits under the skin. Space is getting tight in the uterus at this point, restricting the baby's movements more and more each day.
Week 36 – The baby's gums are very firm right now, almost rigid in some cases. There are still four weeks to go until birth, but some babies will drop into the birth canal at this point. If it doesn't happen this week, it will happen sometime in the next four weeks.
Week 37 – An average size baby will be about 6 ½ pounds or so. The fingers and hands can grasp an object, and there continue to be breathing movements in practice for the real world.
Week 38 – Your baby continues to gain weight, maybe as much as an ounce or more per week. The intestines are really filling up with meconium, too.
Week 39 – The lanugo is nearly all gone now, and the lungs are rapidly maturing. The production of surfactant is increasing quickly to prepare the lungs for successful breathing. Space is extremely tight inside the uterus, making movement quite difficult for your baby.
Week 40 – Wow! Finally, the last week of your pregnancy is here. Your baby is poised and ready to enter the world, weighing on average about 7 ½ pounds and measuring about 20 inches long. Most of the white, cheesy vernix has gone away at this point as well.