Pregnancy is an incredibly busy time for most expectant parents, as they plan for the birth, get ready to welcome a new baby into their lives, and make arrangements for maternity and parental leaves. But the most important part of the pregnancy — the development of the baby inside the mother's body — is one thing you don't have to arrange or plan. Nature takes care of that for you.

Some amazing things are happening inside your body as your baby grows, and many of the most dramatic events have taken place even before you knew you were pregnant. Let's peek inside:

It begins with the joining of a single sperm and an egg. The resulting two-celled zygote contains all the genetic information needed to create this unique baby. From this moment of conception, your baby's gender, eye colour, hair colour and hundreds of other individual characteristics have already been determined. Fertilization normally happens in the mother's fallopian tubes, and the zygote then travels down towards the uterus, dividing into more and more cells along the way.

The trip takes approximately a week, and at the end of the journey the cells will implant in the lining of the uterus (which has been preparing each month just in case this happened!). This tiny clump of cells may not seem very impressive but it is in fact extremely powerful: once implanted in the uterus, the growing baby (now called an embryo) will secrete hormones to stop your menstrual cycle and begin the process of pregnancy. Instead of coming away when your period is due, the lining of the uterus will thicken and eventually the placenta will develop. Your breasts begin to change, too, as the milk ducts develop.

First Trimester: The Blueprint Unfolds
By the time you notice that your period is late, the embryo is beginning to develop specialized cells that will become all the vital organs. A week after your period was due, the heart has formed and is already beating, and the spinal cord is taking shape. These developing organs are sensitive to drugs or infections present in the mother's system — one reason that many physicians encourage women to plan their pregnancies and avoid drugs, alcohol, etc. even before they conceive.

During the next month, your baby will continue to grow at an amazing rate — from about one-eighth of an inch long to one inch long. Still tiny, it is nevertheless beginning to look less like a miniature shrimp and more like a rudimentary human being, with a large head, dark eyes, nostrils, lips, fingers and toes. Eight weeks after conception (or ten weeks pregnant — see "Week What?"), all the major organs have developed, and hair is beginning to grow. Even the teeth have formed as little buds in the jaws. The baby responds to touch and is already moving.

Although you may not look particularly pregnant, the baby growing inside you has had a profound influence on your body. Your cervix is now plugged with thick mucus, your breasts have grown and changed in shape (the nipples may have become darker in colour as well), and you have probably been experiencing tiredness and perhaps nausea. Even your circulatory system has changed to provide increased blood flow to the uterus and nourish the growing baby.

Nine weeks after conception, your baby becomes a fetus. While still small (only 1¼ inches), the baby now looks like a baby, with a big head and skinny arms and legs. Although he or she is very active, you still can't feel any movements. By 12 weeks, the sucking reflex that will be so important after birth has already developed, and some babies have been seen on ultrasounds sucking their thumbs or fingers. The fetus also swallows amniotic fluid and urinates and hiccups. (You'll feel those hiccups later!)

By the end of this trimester, the placenta is fully formed and functioning. The placenta and the umbilical cord provide the essential, life-sustaining link between you and your baby. The umbilical cord carries nutrients and oxygen to the baby, then transports waste products back into your circulatory system.

Second Trimester: The Senses Awaken
At the end of the first 12 weeks, you enter the second trimester of pregnancy. Now you look more pregnant, as your growing uterus pushes out of your pelvis. The nausea and exhaustion of early pregnancy is probably fading, and you may feel more energetic now.

Your baby is feeling pretty energetic, too. Only 3½ inches long at the beginning of this trimester, by 16 weeks after conception she will have grown to 5½ inches, and you may be able to feel her moving (if not, then next month for sure). At first you may feel just a faint fluttering, but soon you will know that this distinctive feeling is your baby, not indigestion or gas.

Although the baby's eyes are still closed, they are sensitive to bright lights, and hearing is quite well developed at this stage. The fetus shows different facial expressions now in response to what is going on in her watery environment, and can make fists or clasp hands together.

At 20 weeks, your nearly eight-inch long baby is probably making you well aware of her presence through kicks, stretches, and somersaults! All the internal organs are formed (including taste buds), but your baby still weighs only about one pound, and would have real trouble surviving if she were born at this stage. The next few months are important as the baby grows bigger, and the lungs, brain and digestive system mature.

Your baby's hearing is particularly acute, and she is beginning to recognize familiar voices - especially mother's and father's. Researchers have discovered that babies can recognize music played to them during the fifth month of pregnancy when they hear it again after birth.

Third Trimester: Preparing For Birth
From now (24 weeks after conception) until birth, your baby is preparing to be born. Layers of fat will be set down under the baby's skin (this helps to regulate his body temperature after birth). The brain grows dramatically (and will continue to grow for the first couple of years after the baby is born). Baby's eyes are open and he can look around inside the uterus. Babies born prematurely at 24 weeks may survive, although their lungs are not mature and they require special medical care.

As your baby gets bigger (usually weighing about 5 pounds by 32 weeks after conception) he has much less room inside the uterus. The baby's movements are less acrobatic, although those kicks can be quite powerful. You may be feeling uncomfortable as your own internal organs are pushed out of the way by your ever-expanding uterus.

The time of your baby's birth is now approaching, and baby is well prepared. He weighs, on average, 7½ pounds (7 if she's a girl), and measures around 20 inches in length. Most critically, his lungs have been practising "breathing" amniotic fluid for some time now and have developed a lining to prevent the tiny tubes from collapsing as baby breathes.

In just nine short months, your baby has grown from two tiny cells into a complex little person, ready to be born and take on the challenges of the world. And although this incredible process happens "all by itself," you play an essential role in this drama, too. Your part? To provide the healthiest possible environment for your growing baby, and to look forward to getting to know him or her when the momentous day of birth arrives.

Week What?
Your Guide To The Countdown

Talking about fetal development is complicated by the fact that doctors date your pregnancy from the first day of your last menstrual period, rather than the date of conception (since that's often not known). So, technically, you were already two weeks' pregnant on the day you conceived! When we talk about the three trimesters of pregnancy, we're referring to roughly three-month periods that correspond to the following "weeks":