Weeks 1 – 2

You’re not really pregnant yet. It’s at this time that the sperm and egg unite in one of your fallopian tubes to form a one-celled entity called a zygote.

Week 3

The zygote is no longer one-celled, but about 500 by now and is called a blastocyst. It has made its way to the uterus and burrow into the uterine wall for nourishment. The placenta, which will nourish your baby throughout the pregnancy, also begins to form.

You may be looking at a positive pregnancy test at this time.

Week 4

The blastocyst is considered an embryo. The baby (now 1/25 of an inch long) is starting to form the brain, spinal cord, heart and other organs.

Week 5

Now your baby’s heart and circulatory system are becoming one, functioning system. About now, your baby’s heart will begin to beat. An ultrasound could even pick up the motion of your baby’s beating heart.

Week 6

Your baby is now about 1/8 of an inch long. The digestive and respiratory systems are beginning to form. Facial features also begin to appear including an opening for the mouth and passageways in the inner ear. This is beginning of the buds will soon grow into arms and legs.

Week 7

The embryo is now considered a fetus. Dark spots on his or her head indicate where the eyes will be forming and pits mark the upcoming nostrils and ears. Your baby’s brain is developing rapidly, and limbs and facial characteristics are now forming. The esophagus and trachea are developing into separate tubes, with lungs positioned on either side.

Week 8

Your baby is no longer a mass of cells, but he’s looking more and more like a miniature baby-to-be. Your baby’s face is becoming more defined: her nose is beginning to protrude, her upper lip is taking shape, and eyelids have formed. Fingers and toes are starting to develop from her arms and legs.

Week 9

Your baby is now nearly 1 inch long and weighs a bit less than 1/8 of an ounce. Your baby’s head — which is nearly half the size of his or her entire body — is now tucked down onto the chest. Hair follicles begin to form. Your baby’s pancreas, bile ducts, gallbladder and anus are in place. The internal reproductive organs, such as testes or ovaries, start to develop, but not the external organs, an ultrasound could not yet tell your baby’s sex.

Your baby may begin moving this week, but you won’t be able to feel it for quite a while yet.

Week 10

Your little one weighs in at a hefty four grams! Your baby’s vital organs have a solid foundation and the bones of your baby’s skeleton begin to form. Your baby has fully separated fingers and toes. Both the external ear and upper lip are complete, and tooth buds are forming as well. The beginnings of external genitalia appear and if your baby is a boy, his testes will start producing the male hormone testosterone.

Week 11

Bigger changes are starting about now until your 20th week (the halfway mark of your pregnancy). Your baby will increase his or her weight 30 times and will about triple in length. The blood vessels in the placenta are growing larger and multiplying so your baby will get enough nutrients. By the end of the week, your baby’s external genitalia will develop into a recognizable penis or clitoris and labia majora.

Week 12

Twelve weeks into your pregnancy, your baby is about the size of a softball. Your care provider will be able to feel your uterus during an abdominal exam. Your baby’s head is nearly half the size of his or her entire body. This week marks the arrival of fingernails and toenails. Your baby’s chin and nose will become more refined as well.

Week 13

Week 13 marks a milestone for your baby-to-be. He’s looking—and acting—more and more like a baby every day. She appears to be resting because her eyelids have formed and are fused together, but her arms and legs move and twitch (although you probably can’t feel it … yet). She can suck her thumb, too.