Our week-by-week illustrated pregnancy calendar is a detailed guide to all the changes taking place in your baby – and in you!

This week‑by‑week pregnancy calendar shows you some of the important developments taking place as your baby grows.

Here’s a peek at what’s going on inside your body during the amazing 40 weeks of pregnancy. Watch your belly — and your baby — grow!

Week 1 is actually your menstrual period, but because your due date is calculated from the first day of your last period, it counts as part of your 40-week pregnancy.

At this point in the pregnancy, all vital organs have been formed and are starting to work together. Congenital abnormalities are unlikely to develop after week 10.

If you saw a picture of your baby now, you’d think you had a genius on your hands — the baby’s head accounts for about half of the body length!

Tiny fingernails and toenails start to form this week, which is the last of your first trimester.

As you begin the second trimester, your baby may be able to put a thumb in his or her mouth although the sucking muscles aren’t completely developed yet.

Some fine hairs, called lanugo, have developed on your baby’s face. This soft colorless hair protects the skin and will eventually cover most of your baby’s body until it is shed just before delivery.

Muscle development continues, and your baby is probably making lots of movements with his or her head, mouth, arms, wrists, hands, legs, and feet.

Your baby can hold his or her head erect, and facial muscles now allow for a variety of expressions, such as squinting and frowning.

Your baby is still very tiny at about 5.1 inches from crown to rump this week.

Your baby’s bones had been developing but were still soft. This week, they begin to harden, or ossify.

A waxy substance called vernix caseosa is covering your baby to help protect the delicate skin from becoming chapped or scratched.

Week 2 is the midpoint of a typical menstrual cycle, when ovulation occurs and conditions are most favorable for fertilization of an egg by sperm.

You’re now halfway through your pregnancy and possibly feeling your baby’s first movements, which may begin between weeks 18 and 20.

Your baby’s intestines are developed enough that small amounts of sugars are absorbed from the fluid your baby swallows and passed through the digestive system to the large bowel.

Brain and nerve endings are formed enough so that the fetus can feel touch, while you might be feeling irregular, painless Braxton Hicks contractions.

You may feel more forceful movements — your baby’s daily workout routine includes moving the muscles in the fingers, toes, arms, and legs.

Your baby’s inner ear has developed enough that your baby can know when he or she is upside down or right side up in your belly.

You may notice that your baby has resting and alert periods. Your baby’s hearing has continued to develop, too — he or she may now be able to hear your voice!

Your baby now weighs a little less than 2 pounds and will gain weight steadily until birth.

This first week of the third trimester, your baby looks similar to what he or she will look like at birth – just smaller and thinner.

Your health care provider may tell you whether your baby is headfirst or feet- or bottom-first (called breech position). Don’t worry if your baby is in the breech position right now — most babies will switch positions on their own.

Your active baby’s first few flutters of movement have given way to hard jabs and kicks that may take your breath away.

During this week, the rapidly dividing fertilized egg will implant itself in the uterus, at the site that eventually will become the placenta.

Your baby continues to gain weight and layers of fat that will provide warmth after birth.

By now, the milk glands in your breasts may have started to make colostrum, the milk that will feed the baby in the first few days if you decide to breastfeed.

At this stage in the pregnancy, your baby’s hair is developing, in the form of eyelashes, eyebrows, and on your baby’s head.

Like a newborn, your baby sleeps much of the time and even has REM sleep, the stage when our most vivid dreams happen.

Your baby is probably in position for delivery — your health care provider can tell you if your baby is positioned head- or bottom-first.

Because of increasing size, your baby is now cramped and restricted inside the uterus. Fetal movements may decrease, but feel stronger and more forceful.

At this point, your appetite may return because the baby has dropped down into your pelvis, and is no longer putting as much pressure on your stomach and intestines.

This week, your baby continues to gain weight — at half an ounce a day!

At this point, you may be taking frequent trips to the bathroom. That’s because your baby is engaged in your pelvis, so your bladder is extremely compressed.

Braxton Hicks contractions (also called “false labor”) may become more pronounced, and your water may break.

During this week, your baby, or embryo, has two layers of cells that will develop into organs and body parts.

Your baby is here! Or maybe not — most women, especially first-time moms, don’t deliver on their estimated due dates.

This week, the embryo begins to form a distinct shape that includes the neural tube, which will become the spinal cord and brain.

Your baby’s heart will begin to beat around this time, and the beginnings of the digestive and respiratory systems are forming, as are small buds that will grow into arms and legs.

The umbilical cord has formed, and the mouth, nostrils, ears, and eyes are some of the facial features that become more defined this week. The arm bud now has a hand on the end of it, which looks like a tiny paddle.

Around this time, symptoms such as a missed period, nausea, extreme fatigue, or tight clothes may make the reality of pregnancy hit home.

Your baby measures about 0.6 to 0.7 inches from crown to rump and weighs around 0.1 ounces. Your baby may make some first movements as muscles develop, but you won’t feel them for several more weeks.