First Trimester | Week 1:

The forty weeks of pregnancy are counted from the beginning of the last menstrual period prior to the pregnancy. This means that for the first two weeks,  conception has not yet taken place. When you are trying to get pregnant,  or are having sexual intercourse without taking steps to prevent pregnancy,  you should take care to stay as healthy as possible so that your future baby will have every chance to develop normally. Eat a healthy diet,  and take a daily prenatal vitamin that contains at least 0.4 to 0.8 milligrams of folic acid. This will help lower your baby, s risk of neural tube defects such as spina-bifida.

First Trimester | Week 2:

Although this is counted as the second week of pregnancy,  conception still has not taken place. You have not yet ovulated during this cycle. By the end of this week,  the egg will be released by the ovary and enter the fallopian tube. The lining of your uterus gets ready to receive a fertilized egg,  and your body begins to secrete a hormone (FSH) that helps the egg mature. This is the week in which conception is most likely to occur. If you have sexual intercourse during this week and do not practice any method of birth control,  the chance of becoming pregnancy is high.

First Trimester | Week 3:

If a sperm was successful in fertilizing the egg,  this is the week when you actually become pregnant. The fertilized egg (ovum) has already divided into hundreds of cells. It is now a blastocyst. ( Definition: Blastocyst the hollow ball stage of embryonic development.) This week,  if the pregnancy is going to be successful,  the blatocyst,  after a journey of 7-10 days through the fallopian tube, will implant itself in the lining of the uterus (endometrium) The place where the blastocyst implants will later grow into the placenta,  the organ that will nourish your baby via his umbilical cord until birth.
 

First Trimester | Week 4:

This week the gestational sac is formed. The group of cells that began to divide at fertilization is now officially called an embryo. The cells will divide into the group that forms the baby and the group that forms the placenta,  and implantation into the uterine lining will be complete.
Chances are that you do not yet know that you are pregnant. Even so,  these very early stages of pregnancy are extremely important to your developing baby. You should not smoke or even breathe in second-hand smoke; drinking alcohol or using drugs at this time can be very harmful to the baby. If you take prescription or over-the counter medications and are trying to become pregnant,  check with your doctor about possible effects of these drugs.
 

First Trimester | Week 5:

Your period has not yet arrived and you wonder if you might be pregnant. This is the time to take a home pregnancy test. Some women experience light spotting at the time of the first missed period; if this happens, or your pregnancy test is inconclusive,  you might want to make an appointment with your health care provider to take a more accurate blood test to determine pregnancy. This week, the embryo begins to form a head end and a tail end, and its heart will begin beating even though the heartbeat cannot yet be picked up by the doctor. Everything that will make your baby who he/she is, coloring, gender, genetic characteristics, are already present in this tiny being who is now between 1 1/2 to 2 1/2 mm long.

First Trimester | Week 6:

Lots is happening this week; your baby is in a period of very rapid growth. His brain and spinal column are beginning to appear and grow. His eyes inner ear and voice box begin to form, and tiny buds appear that will become arms and legs. Blood begins to circulate in his tiny system. He grows to between 4 and 6 mm in length.
 

First Trimester | Week 7:

In spite of the very rapid growth, and the development of so many parts of his body, your baby is still barely visible to the eye, (about ¼ to 1/2 inches long), and he weighs a little less than a gram. Handplates and genital tubercles appear, though it is not yet possible to tell his sex.
 

First Trimester | Week 8:

Your baby's brain and gonads are growing fast, and the little embryo begins to move independently, although you cannot yet feel his movement. It is time to make an appointment with your doctor for a prenatal visit.
 

First Trimester | Week 9:

By now your baby is about 1 inch long (22-30 mm), and weighs about 4 gm. His toes begin to form, his bones begin to harden, and his gonads (testes or ovaries) are full formed.

First Trimester | Week 10:

Your baby is now a fetus. By the end of this week, all of his organs should be formed and should begin to function. Eyes, ears, lips and genitals are now visible. The baby should be nearly 1 1/2 inches in length at the end of this week.

First Trimester | Week 11:

Your baby's head makes up half the length of his body at this point. Tiny fingernails are already beginning to grow. He weighs about 7 gm.

First Trimester | Week 12:

Your baby's digestive tract is now formed and active. His heartbeat can be picked up with a Doppler at your doctor's visit. You have probably been experiencing nausea and even vomiting during the past few weeks (morning sickness), but this should be nearing an end. Your are nearing the end of the first trimester of your pregnancy.
 

Second Trimester | Week 13:

Your baby now weighs about 1 oz (30 gm). Teeth buds are forming in his gums, and the villi in his intestines begin to function.

Second Trimester | Week 15:

Your baby's skin now covers his tiny body, and it is translucent so that you can see blood vessels through the skin. Hair grows on his head, and his eyebrows begin to grow. He should now weigh over 2 oz (70 gm).

Second Trimester | Week 16:

At this point in your pregnancy, your baby is fully formed and all of his systems have begun to function. You will probably begin to feel the baby's movement like tiny "butterfly wings" or "bubbles." It might be possible to tell the sex of the baby if you have an ultrasound at this time.

Second Trimester | Week 17:

Your baby is now over 5 inches long and weighs nearly 5 oz. The first layers of brown fat, which will help to control the baby, s temperature when he is born, are now forming. The baby is now larger than the placenta, and both baby and placenta will continue to grow at a rapid rate.
 

Second Trimester | Week 18:

Your baby's bones are continuing to harden, and finger and toe pads begin to form. Soon he will have fingerprints. The ears have developed enough that your baby will begin to hear the sounds of your body and your heartbeat. Loud noises may startle him, He is now about 8 in. long and weighs about 7 oz.

Second Trimester | Week 19:

The baby now weighs 8 oz or about 1/2 pound. Permanent teeth buds are forming in the gums. A baby girl's ovaries already contain the primitive cells of the eggs that they will produce when she reaches maturity. Fine hair called lanugo forms and covers the baby's body.
 

Second Trimester | Week 20:

This is the halfway mark of your pregnancy. Your baby is over 6 inches long and weighs about 10 1/2 oz. A cheesy substance called vernix begins to form on his skin. It will protect him as he floats in the amniotic fluid, and will still be visible at birth.

Second Trimester | Week 21:

Your baby is now swimming in the amniotic fluid that surrounds him in his sac. This is sometimes called the "bag of waters." It protects the growing fetus from injury and from germs. He begins to swallow tiny amounts of this fluid that surrounds him and to absorb some of the sugars that are present in the fluid. The placenta continues to be his main source of nourishment.
 

Second Trimester | Week 22:

Your baby now weighs about a pound and is 11 inches long. This is a time of very rapid growth for your baby's brain. He can touch and feel, and his taste buds begin to form. He experiments by stroking his face and may suck his thumb. At this stage in your pregnancy, you may begin to feel occasional Braxton-Hicks contractions, occasional tightening sensations in the top of your uterus which are like practice sessions for labor. Regular or painful contractions should be reported to your doctor, because they may be a sign of premature labor.
 

Second Trimester | Week 23:

Meconium begins to form in your baby's intestines as a result of his swallowing the amniotic fluid. This first stool will be passed after he is born. As he swims in the fluid he exercises arms and legs, fingers and toes, so that you probably become aware of much more movement. Call your doctor if you find that your baby is less active that normal or that his movement has decreased.
 

Second Trimester | Week 24:

Your baby now weighs about 1 lb, 5 oz. And is nearly 12 in. long. His lungs are still very immature, but they now begin to produce surfactant, a substance that will help them prepare to function during breathing once he is born. Some babies born at this stage of development are able to survive, although they require months of intensive care in a N.I.C.U.

Second Trimester | Week 25:

Your baby's bones are continuing to harden. He has more control over his fingers and hands, and he may play with his umbilical cord. He now has times when he is more wakeful, and times when he is quiet.
 

Second Trimester | Week 26:

By now your baby weighs about 2 lbs and is about 14 inches long. His hearing is well developed, and he may jump when hears a loud noise. His eyes open and he begins to blink. Eyelashes begin to grow in.
 

Third Trimester | Week 27:

Your baby's brain is developing at a rapid rate, and his hearing is at a point where he can begin to hear your voice. His skin is very wrinkled from the fluid that surrounds him. Babies born at this stage of development have an 85% survival rate, but would still need months of N.I.C.U care. You are now officially entering the third trimester of your pregnancy.

Third Trimester | Week 28:

Your baby now weighs around 2 lbs, 2 oz. And he is about 15 inches long. Subcutaneous fat is being deposited, and a baby boy's testicles are descending from the abdomen into the scrotum.

 

Third Trimester | Week 29:

At this stage, the baby's head is in the same proportion to the rest of his body as it will be when he is born. You now feel his movements as hard kicks or jabs. During his active periods your baby should kick or move about ten times each hour. Notify your doctor if you do not feel frequent movements or if you have other concerns. Your baby's kidneys are now functioning, and he urinates about 1/2 quart of urine every day into the amniotic fluid.

Third Trimester | Week 30:

Your baby now weighs close to 3 lbs and is nearly 15 1/2 inches long. He begins practicing the movements of breathing, and you may notice that he often has the hiccups. He can distinguish light from dark, and his sleep and wake cycles may become more noticeable.
 

Third Trimester | Week 31:

Your baby now looks very much like he will look at birth, and weighs about 3 lbs, 5 oz. His skin is pink and all of his systems are functioning. If he is born at this time, he has an excellent chance of surviving with good hospital care.

Third Trimester | Week 32:

By now your baby weighs around 4 lbs. His fingernails and toenails are fully present, and his hair, eyebrows and eyelashes are visible. His ability to suck is developing, and he continues to gain deposits of fat under the skin.

Third Trimester | Week 33:

As you approach your due date, the baby continues to gain weight and his lungs and nervous system continue to mature. Surfactant is now present in good amounts, decreasing the chance of breathing difficulty if your baby is born after this week.
 

Third Trimester | Week 34:

Your baby continues to grow, and the stores of fat deposited under his skin continue to increase. He experiences long periods of sleep, and even has the type of REM sleep where dreams can be experienced. He now weighs about 4 1/2 lbs and is over 16 inches long. Your body is getting prepared for labor.
 

Third Trimester | Week 35:

Your baby may weigh over 5 lbs by this time, and he is close to 18 in. long. Space is getting very tight inside your uterus, and most babies will be in the normal head down position for birth. From this point on, most babies will gain between 1/2 and ¾ lbs per week until they are born.
 

Third Trimester | Week 36:

Your baby probably weighs close to six lbs by this time, and the fat pads on his cheeks are developed as is his strong sucking reflex. He is settling further down into the birth canal. With a month or less to go, it is time to get everything ready for your baby's birth.
 

Third Trimester | Week 37:

The baby is still experiencing rapid growth, and now weigh about 6 1/2 lbs. He continues to practice breathing movements and will turn towards a light source, preferring that to total darkness. About 4% of babies have settled into a breech position by this time; if your baby is breech, the doctor may attempt to turn him into a head down position.

Third Trimester | Week 38:

From this point on, your baby is considered full term. If labor begins, no attempt would be made to stop it, and your baby would not be premature, even though he were born a little early.  Everything is functioning and ready to go!
 

Third Trimester | Week 39:

From this point on, the baby continues to gain weight and to grow, but all of his systems should be fully mature and ready to function outside the womb. Many babies will be born at this week of gestation. By now your suitcase should be packed, your birthing partner on 24 hour alert, and your are probably getting very anxious to meet your baby.

Third Trimester | Week 40:

Your baby is probably ready to be born, although very few babies come exactly on their due date. Although all of his systems are functioning well, his immune system is still immature. Antibodies from the placenta protect him from germs, and the antibodies in your colostrum (first milk) will provide important protection from the organisms that will surround him once he leaves the safety of the womb and enters the world. The average newborn weighs about 7 1/2 lbs and is 20 in. long.