Pregnancy — when a sperm and an egg join to form a single cell — is the starting point for an extraordinary chain of events. That microscopic cell divides and redivides, repeatedly, until, in about 9 months, it has changed into about 2 trillion cells of different varieties, all intricately packaged as a new person — your one-of-a-kind baby.

During this time, your body prepares to provide the nourishment and hormones that govern your baby's growth and development. A preset genetic timetable determines what happens and when as your body undergoes significant changes throughout your pregnancy.

If this is your first baby, nothing that you've read or heard quite prepares you for all of the experiences of pregnancy. It may be exciting and uneventful, anxiety-producing and deeply satisfying — all at the same time. You'll probably experience new, unexpected sensations, some comforting, others unsettling. Here's a look at some common changes to expect during your pregnancy.

During the first 3 months of pregnancy (first trimester), your baby's major organs — heart, lungs, brain — begin forming. About 2 weeks after the developing egg embeds itself in the lining of your uterus, your body begins to change, even before tests and a doctor's examination can confirm that you're pregnant. At this early time, your body starts preparing itself for your baby's arrival. Some of the first changes, induced by hormones from the fetus and your own body, involve:

• Menstrual flow. Contrary to popular belief, you may not entirely miss your first menstrual period. Spotting or a scanty menstrual flow may be your first indication of pregnancy. This spotting doesn't resemble a normal menstrual period; it comes from a small amount of bleeding that can occur when the developing egg implants into the lining of your uterus.
• Breasts. Often the first hint of pregnancy is a change in the way your breasts feel. They may be more sensitive and tender. They may feel fuller and heavier. As early as 2 weeks after conception, your breasts and nipples may start to enlarge. The rings of brown or reddish skin around the nipples (areolas) enlarge and darken.
• Fatigue. Most, if not all, women are more tired than usual during early pregnancy. During the first weeks of pregnancy, your body produces more blood to carry nutrients to the fetus. Your heart must increase its efforts to handle this increased blood flow. Your body changes the way it uses water, protein, carbohydrates and fat. The combination of these profound changes may play a role in the fatigue that you feel.
• Morning sickness. Morning sickness is the blanket term for the queasiness, nausea or vomiting that many pregnant women experience during the first 12 to 14 weeks. It tends to be worse in the morning, but some women feel nauseated throughout the day, especially when their stomach is empty. Fortunately, morning sickness seldom leads to significant weight loss or dehydration. It's unclear exactly why morning sickness occurs. It may be related to hormonal or gastrointestinal changes, and it may be aggravated by the emotional stress and fatigue that often accompany pregnancy.
• Increased urination. The increasing size of the uterus in the first trimester, along with increased functioning of the kidneys, may cause you to feel the need to urinate more often. You may also leak urine when sneezing, coughing or laughing. This is due to the enlarging uterus pressing against your bladder.
• Weight gain. Although you'll probably gain about 25 to 30 pounds during your pregnancy, you'll put on only a small percentage of this amount during the first trimester. In fact, a typical weight gain during the first trimester is about 2 pounds.
• Uterus. Within 2 weeks after conception, your uterus begins to change. Its lining thickens. Blood vessels in the lining enlarge to nourish the growing fetus. If you've never been pregnant, your uterus is about the size of a small pear. During pregnancy, its capacity expands to accommodate your fully developed baby before delivery.
• Cervix. Your cervix, the opening of your uterus through which your baby will emerge, begins to soften. Your doctor looks for this change during your first examination as an indication of pregnancy.

During the second 3 months (second trimester), your baby grows from about 4 inches to nearly a foot in length. Your uterus expands and begins to displace other organs in your abdomen. Your baby probably makes itself known with beginning movements — called quickening — and, by the sixth month, usually with definite kicks. For many women, these months probably are the easiest of pregnancy — they've moved beyond the first trimester morning sickness, and have yet to experience the increasing demands that the baby puts on their body during the third trimester.

You may experience changes in some or all of the following during your second trimester:

Breasts. Stimulated by estrogen and progesterone, your breasts enlarge as the milk-producing glands inside them increase in size. Breast enlargement accounts for about a pound of the weight you gain during your pregnancy. Some, but not much, of the increased breast size is due to increased fat. The tiny bumps that circle the areolas — skin glands called Montgomery's tubercles — start enlarging. These glands secrete oils that moisturize and soften the skin of your nipples and areolas.

Uterus. As your uterus expands to accommodate your growing baby, so does your abdomen — week by week. Up to the 12th week, your uterus fits neatly inside your pelvis. By the 20th week, your uterus reaches your navel. Ultimately it extends from your pubic area to the bottom of your rib cage.

Skin. You've probably heard about the glow of pregnancy. Blood circulation increases during pregnancy, including the tiny vessels just beneath the surface of your skin. The result may be a healthy glow.

Palms and soles. Your palms and the soles of your feet are likely to become red and itchy. It's probably caused by the increase in estrogen. Like most skin changes, the redness fades after delivery.

Nails. Your fingernails and toenails may grow faster than usual. They may become brittle or soft and grooved, partly under the influence of pregnancy hormones and occasionally because of anemia.

Eyes. Your body retains extra fluid throughout pregnancy. This causes the cornea — the outer layer of your eye — to get thicker, a change that may become apparent by about the 10th week of pregnancy and persist until about 6 weeks after delivery. The pressure of fluid within your eyeball (intraocular pressure) decreases during pregnancy. These two events, in combination, may cause slightly blurred vision.

If you wear contact lenses, particularly hard lenses, you may find them uncomfortable because of these changes. But don't change your contact lenses during pregnancy. Your vision returns to its normal state after you give birth.

Mouth. Pregnancy does not cause cavities in your teeth, as is commonly believed. However, increased blood circulation may make your gums softer. A majority of pregnant women experience this softening, which may cause minor bleeding when brushing your teeth. If the problem persists, see your doctor or dentist.

During the final 3 months (third trimester), your baby grows more rapidly, gaining about one-half pound a week and reaching about 20 inches in length. You usually feel some kicks and even get a sense of how the baby is positioned. Your baby's growth may cause increased frequency of urination and shortness of breath, as your uterus expands beneath the muscle just below your lungs (diaphragm). As your body further prepares for birth, you may feel early contractions, your baby moves lower in your uterus, and the opening at the neck of your uterus (cervix) softens.

You may experience changes involving some or all of the following during your third trimester:

Abdomen. Your expanding uterus presses on many other organs — bladder, kidneys, stomach, intestines, diaphragm and blood vessels in your abdomen — affecting these organs' functions. Toward the end of pregnancy, your protruding abdomen may sag lower. You may feel your baby settling down in your pelvis in preparation for birth.

Stretch marks. Some women are prone to develop stretch marks — pink, reddish or purplish indented streaks in the skin of the abdomen, breasts, upper arms, buttocks and thighs. Stretch marks are not a sign of excessive weight gain. They seem to be caused, literally, by a stretching of the skin coupled with a normal increase in cortisone, a hormone produced by the adrenal glands, which may weaken elastic fibers of the skin. Fortunately, stretch marks often fade to light pink to grayish stripes, but they usually don't disappear completely.

Acne. If you commonly had experienced skin break-outs during your menstrual periods, you may develop acne early in your pregnancy. Or your acne may actually improve during pregnancy. The increase in progesterone, which stimulates oil secretion from skin glands, may explain why some women are prone to acne during pregnancy.

Skin pigmentation. It's common to notice changes in the color or pigmentation of the skin on your cheeks, chin, nose and forehead. These areas may grow darker as a result of increased amounts of estrogen and progesterone. You're likely to notice that areas of skin that are already pigmented get even darker, most noticeably the areolas surrounding your nipples and the labia, the double folds of tissue on both sides of your vaginal opening. Although some of this increased pigmentation fades after you have given birth, these areas are likely to remain darker than before you were pregnant.

Melasma. Sometimes referred to as chloasma, this brownish darkening of the facial skin has been called the mask of pregnancy. It primarily affects women who are dark haired and fair skinned. Melasma usually appears on the forehead, the temples and the central part of the face. It may not be as intense as other increases in pigmentation and generally fades completely after delivery. Melasma may be caused or aggravated by exposure to sunlight.

Itchiness. Many pregnant women experience itchiness on their abdomens, or all over their bodies. Such itchiness typically vanishes spontaneously. Moisturizing your skin usually helps. Also, if you have itchiness, avoid getting overheated. Anything that itches usually itches more if you're warm. If these measures don't provide relief, your doctor may prescribe a medication or ultraviolet light therapy.

Cholestasis of pregnancy. Cholestasis, a reduction in the flow of bile, may indicate altered liver function related to pregnancy and can cause severe itching and even nausea, vomiting, loss of appetite, fatigue and possibly jaundice — a yellowing of the skin. If severe itching develops late in your pregnancy, your doctor may order blood tests to check your liver. Prescription medications may help relieve the symptoms, but usually this problem goes away after you give birth.

Vascular "spiders." Minute reddish spots with tiny blood vessels protruding outward — like spider legs — occur in many pregnant white women. Caused by increased blood circulation, and perhaps increased estrogen, vascular spiders may appear on your face, neck, upper chest or arms. They usually disappear within a few weeks after birth.

Bluish and blotchy legs. Especially in cold temperatures, a temporary skin discoloration caused by increased estrogen production develops in some women. Don't worry about it. This disappears after your baby is born.

Varicose veins. Some pregnant women develop protruding, bluish veins, particularly in their legs. Veins throughout your body become larger during pregnancy to accommodate increased blood volume. This change is especially noticeable in veins near the skin surface, as in your legs. Varicose veins are usually not serious, but they can be uncomfortable and may cause sore, aching legs.

Perspiration and heat rashes. Pregnant women often note increased perspiration because of the action of hormones on sweat glands spread throughout their entire body. This makes heat rashes more common. Oddly, perspiration in the areas under the arms, breasts and genital area is less frequent during pregnancy. You may notice more heat rash but less body odor while you're pregnant.

Puffiness of the face. During the last 3 months of pregnancy, many women notice that their eyelids and face become puffy, usually in the morning. Some puffiness is due simply to increased blood circulation. But if your eyelids are puffy along with a sudden weight increase — 5 pounds or more within a week — contact your doctor. Sudden weight gain and puffiness could indicate excessive fluid retention, which is often coupled with high blood pressure.

Hair. Later in pregnancy, you may find yourself developing a more luxuriant head of hair than ever before. After pregnancy, you may temporarily lose more hair than normal.

When you're not pregnant, each of the 100,000 or so hairs on your head grows about a half an inch a month for periods of 6 to 8 years. Then they go into a resting phase, stop growing, and gradually fall out at a rate of about 100 hairs a day, mostly during brushing or washing, until the growth cycle starts again. During pregnancy, hair growth tends to remain in the resting phase longer. Because fewer hairs fall out each day, you may have a fuller head of hair.

After delivery, the resting phase shortens. More hairs fall out, and your hair follicles start to grow new hairs. Around 6 to 12 weeks after birth, you may notice a dramatic increase in hair shedding. For a few months, your hair may feel thinner. But within 6 to 15 months, you'll likely return to the same head of hair you had before.

Some women, especially those who already have a fair amount of body hair, develop more hair during pregnancy. Hormones produced by the placenta and increased cortisone levels stimulate blood circulation to your hair follicles. Hair growth may be especially noticeable on your face and extremities. This excess hair growth usually decreases in about 6 months, though it may recur in subsequent pregnancies.