In-Utero Guide

By Kristine Ziwica for Your Baby Today

Pregnancy is indeed a scientific marvel: A cluster of cells evolves into a new life in just 280 days. But if you're an expectant parent, you're no doubt wondering, "What's going on in there?" Find the answers below with a week-by- week guide to your baby's fetal development. (Since doctors consider pregnancy a 40-week affair starting with the first day of your last period, your pregnancy really "begins" two weeks before you conceive.)

Week 1 | Week 2 | Week 3 | Week 4 | Week 5 | Week 6 | Week 7 | Week 8 | Week 9 | Week 10 | Week 11 | Week 12 | Week 13 | Week 14 | Week 15 | Week 16 | Week 17 | Week 18 | Week 19 | Week 20 | Week 21 | Week 22 | Week 23 | Week 24 | Weeks 25 through 29 | Weeks 30 through 34 | Weeks 35 through 40

Week 1:

You get your period.

Week 2:

While an egg ripens in one of your two fallopian tubes, the uterus starts to form a lining of blood-rich tissue.

Week 3:

Conception. Only one of the thousands of sperm that have traveled through the fallopian tubes breaks through the egg's protective barrier to form a zygote. The cells start to divide into a small cluster as they travel down the fallopian tube towards the uterus.

Size: .006 inches

Week 4:

The tiny cluster of cells attaches itself to the uterine wall and then divides in half. One half becomes the placenta and the other half the baby. At the same time, different germ layers form. These are the building blocks for the central nervous and skeletal systems.

Size: .04 inches

Week 5:

This is a week of crucial wiring when the placenta and umbilical cord hook up to Mom. Your baby's heart -- about the size of a poppy seed -- starts beating.

Size: .05 inches

Week 6:

The cluster of cells takes on the shape of a baby with a prominent head and dark eyespots. The vital organs including the brain, spinal cord, heart, kidneys, liver, and stomach also start to form. With all this activity, week 6 marks the beginning of a vital 4-week embryonic period (the time when most malformations can happen.)

Size: .08 to .16 inches

Week 7:

With your child's biggest growth spurt until puberty, the embryo more than doubles in size from .16 to .44 inches. The legs and arms extend and grow from the torso and the heart divides into right and left chambers. The forebrain also divides into two hemispheres.

Size: .44 to .52

Week 8:

Finite features including eyelids, ears, hair, fingers, and toes start to take shape.

Size: .56 to .8 inches

Week 9:

The eyelids now cover the eyes and will remain sealed shut until week 27. Pupils form and as the arms and legs grow longer, your future Olympian can practice some early moves. But since the baby still has plenty of room in the uterus, you won't feel these movements for several more weeks.

Size: 09 to 1.2 inches

Week 10:

About the size and shape of a medium shrimp, week ten marks the end of the embryonic period and the embryo becomes a fetus with all vital organs and weighs in at about .18 ounces.

Size: 1.25 to 1.68 inches

Week 11:

The baby's body isn't very proportionate and the head makes up half the size. The neck develops and the head rises up from the chest. It may be possible for curious parents to tell if they have a boy or girl.

Size: 1.75 to 2.4 inches Weight: .3 ounces

Week 12:

The baby takes up more space and the uterus expands up from the pelvic floor and pushes other organs out of the way. If you could peek into the womb, you would see quite a busy little fellow: He's now squinting, opening and closing his mouth, and moving his fingers and toes.The small intestine starts functioning, relaxes and contracts to push substances out. Few structures form after this point. Those already in place continue to grow and develop.

Size: 2.5 inches Weight: .3 to .5 ounces

Week 13:

About the size of a peach, your baby's body starts to catch up to the head. Eyes, which start out on the side of the face, move toward the center and the ears move toward the side of the head. The genitalia is developed enough to easily tell whether you have a boy or girl.

Size: 2.6 to 3.1 inches Weight: .5-.7 ounces

Week 14:

The neck gets longer and pulls the head further up away from the chest.

Size: 3.2 to 4.1 inches Weight: 1 ounce

Week 15:

The skeletal system is getting harder as your baby takes in nutrients from Mom, namely calcium. If you did a x-ray or even looked through the baby's very thin skin, you would see the outline of a skeleton. Fine hairs called lanugo also cover the body.

Size: 4.1 to 4.5 inches Weight: 1.75 ounces

Week 16:

The baby continues to grow and take up more space in the uterus while showing a decided flair for movement. Less space means an elbow here, a foot there. You, therefore, may experience movements for the first time, called fluttering. (This can take place anytime through week 20. Notify your doctor if you don't notice them by then.)

Size: 4.3 to 4.6 inches Weight: 2.8 ounces

Week 17:

While most of your baby's organs are in place, he's still very thin and lanky. During week 17 your baby starts to form fat. This is vital for the baby's heat production and metabolism.

Size: 4.4 to 4.8 inches Weight: 3.5 ounces

Week 18:

The rapid growth rate of previous weeks slows down. Finite features of the heart including ventricles and chambers should be visible during an utltrasound and some abnormalities can be detected.

Size: 5 to 5.6 inches Weight: 5.25 ounces

Week 19:

The sebaceous glands start producing vernix caseosa, a covering that protects the delicate fetal skin from the amniotic fluid. Myelination of the spinal cord also takes place.

Size: 5.2 to 6 inches Weight: 7 ounces

Week 20:

As they say in the baby business: You've reached the halfway point. By now the fetus is so large, you notice most movements. The skin also forms two layers: the epidermis and the dermis.

Size: 5.6 to 6.4 inches Weight: 9 ounces

Week 21:

The different organ systems developed during the crucial embryonic phase start to perform their duties, prepping the baby for survival outside the womb. During this week, your baby may start swallowing small amounts of amniotic fluid, absorbing most of the water, and passing the rest through to the large bowel.

Size: 7.2 inches Weight: 10.5 ounces

Week 22:

Eyelids, eyebrows, and even fingernails have developed. By now your baby's liver, with some help from Mom, is starting to break down bilirubin, a substance produced by red blood cells. If your baby can not sufficiently break down this material by birth, he might be slightly jaundiced and need to be placed under lights.

Size: 7.6 inches Weight: 12.25 ounces

Week 23:

Your baby starts practicing breathing by taking in and expelling small amounts of amniotic fluid. The pancreas also produces insulin, important for the breakdown of sugars. By now your baby resembles a small doll with all the facial features of an infant at birth.

Size: 8 inches Weight: Almost 1 pound

Week 24:

With all of the rapid organ development, and in-womb practice, your baby may be able to survive outside the womb. (Week 25 remains the official viability week.) The baby's once wrinkled skin has filled out with weight gain, and there is little space left inside the womb.

Size: 8.4 inches Weight: 1.2 pounds

Weeks 25 through 29:

By this time your baby has a pretty good chance of surviving outside the womb. The eyes open and the retina forms different layers which can sense light levels and send messages to the brain. Also, the once smooth surface of the brain starts to form grooves and indentations.

Size: 8.8 inches to 10.4 inches Weight: 1.5 pounds to 2.7 pounds

Weeks 30 through 34:

While continuing to plump up, the baby is sensitive to the world around him, responding to light, sounds, and making faces.

Size: 11.2 inches to 12.8 inches Weight: 3.5 pounds to 5 pounds

Weeks 35 through 40:

In anticipation of delivery, the baby positions itself with the head down towards the pelvis and sheds lanugo hairs. At any point between week 35 and 40, your little one will let you know he's ready to make a grand entrance and give you your first glimpse at 40 weeks of amazing development.

Size: ? Weight: ?