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A male sperm penetrates an egg and fertilizes it. This usually happens in the woman's fallopian tube after ovulation (the matured egg was released from one of her ovaries).
Sperm has 23 chromosomes, including one of two types of sex chromosomes - X chromosome or Y chromosome. A sperm with an X chromosome that fertilizes an egg will produce a girl, while a Y chromosome sperm will produce a boy. Each egg also has 23 chromosomes. 46 chromosomes are required to make a human. The fusion of sperm and egg makes 46.
As soon as it is fertilized, the zygote starts to divide; it continues doing so until there is a cluster of cells. After five to seven days of dividing and growing, it attaches itself to the wall of the uterus (womb) and puts out root-like veins called villi. The villi make sure the embryo is well anchored to the lining of the uterus. These villi will eventually become the placenta, which feeds and protects the embryo/fetus.
Through the placenta the embryo/fetus receives its oxygen and nutrition, and expels waste.
The earliest symptoms of pregnancy can appear in the first few weeks after conception. Here's what you may experience, from nausea and tender breasts to dizziness and mood swings.
Are you pregnant? The proof is in the pregnancy test. But even before you miss a period, you might suspect that you're pregnant. For some women, early symptoms of pregnancy begin in the first few weeks after conception.
In addition to a missed period, the earliest symptoms of pregnancy might include:
Nausea with or without vomiting. Morning sickness, which can strike at any time of the day or night, sometimes begins as early as three weeks after conception. Nausea seems to stem at least in part from rapidly rising levels of estrogen and progesterone, which cause the stomach to empty more slowly. Pregnant women also have a heightened sense of smell, so various odors — such as foods cooking, perfume or cigarette smoke — might cause waves of nausea in early pregnancy.
Tender, swollen breasts. Your breasts might provide one of the first symptoms of pregnancy. As early as two to three weeks after conception, hormonal changes might make your breasts tender, tingly or sore. Or your breasts might feel fuller and heavier.
Increased urination. You might find yourself urinating more often than usual, especially at night.
Fatigue. Fatigue also ranks high among early symptoms of pregnancy. During early pregnancy, levels of the hormone progesterone soar — which can put you to sleep. At the same time, lower blood sugar levels, lower blood pressure and increased blood production might team up to sap your energy.
Food aversions or cravings. When you're pregnant, you might find yourself turning up your nose at certain foods, such as coffee or fried foods. Food cravings are common, too. Like most other symptoms of pregnancy, these food preferences can be chalked up to hormonal changes — especially in the first trimester, when hormonal changes are the most dramatic.
Sometimes symptoms of pregnancy are less familiar or obvious. If you're pregnant, you might experience:
Slight bleeding or cramping. Sometimes a small amount of spotting or vaginal bleeding is one of the first symptoms of pregnancy. Known as implantation bleeding, it happens when the fertilized egg attaches to the lining of the uterus — about 10 to 14 days after fertilization. This type of bleeding is usually a bit earlier, spottier and lighter in color than a normal period and doesn't last as long. Some women also experience abdominal cramping — similar to menstrual cramping — early in pregnancy.
Mood swings. The flood of hormones in your body in early pregnancy can make you unusually emotional and weepy. Mood swings also are common, especially in the first trimester.
Dizziness. As your blood vessels dilate and your blood pressure drops, you might feel lightheaded or dizzy. Early in pregnancy, faintness also can be triggered by low blood sugar.
Constipation. An increase in progesterone causes food to pass more slowly through the intestines, which can lead to constipation early in pregnancy. Constipation can be aggravated by prenatal vitamins containing iron.
In addition, your basal body temperature — your oral temperature when you first wake up in the morning — might provide an early clue about pregnancy. Basal body temperature increases slightly soon after ovulation and remains at that level until your next period. If you've been charting your basal body temperature to determine when you ovulate, its continued elevation for more than two weeks could mean that you're pregnant.
Unfortunately, these signs and symptoms aren't unique to pregnancy. Some can indicate that you're getting sick or that your period is about to start. Likewise, you can be pregnant without experiencing any of these signs and symptoms.
Still, if you miss a period or notice any of the tip-offs on these lists, you might want to take a home pregnancy test — especially if you're not keeping track of your menstrual cycle or if it varies widely from one month to the next. If your home pregnancy test is positive, make an appointment with your health care provider. The sooner your pregnancy is confirmed, the sooner you can begin prenatal care.
During the first trimester your body undergoes many changes. Hormonal changes affect almost every organ system in your body. These changes can trigger symptoms even in the very first weeks of pregnancy. Your period stopping is a clear sign that you are pregnant. Other changes may include:
Extreme tiredness
Tender, swollen breasts. Your nipples might also stick out.
Upset stomach with or without throwing up (morning sickness)
Cravings or distaste for certain foods
Mood swings
Constipation (trouble having bowel movements)
Need to pass urine more often
Headache
Heartburn
Weight gain or loss
As your body changes, you might need to make changes to your daily routine, such as going to bed earlier or eating frequent, small meals. Fortunately, most of these discomforts will go away as your pregnancy progresses. And some women might not feel any discomfort at all! If you have been pregnant before, you might feel differently this time around. Just as each woman is different, so is each pregnancy.
Most women find the second trimester of pregnancy easier than the first. But it is just as important to stay informed about your pregnancy during these months.
You might notice that symptoms like nausea and fatigue are going away. But other new, more noticeable changes to your body are now happening. Your abdomen will expand as the baby continues to grow. And before this trimester is over, you will feel your baby beginning to move!
As your body changes to make room for your growing baby, you may have:
Body aches, such as back, abdomen, groin, or thigh pain
Stretch marks on your abdomen, breasts, thighs, or buttocks
Darkening of the skin around your nipples
A line on the skin running from belly button to pubic hairline
Patches of darker skin, usually over the cheeks, forehead, nose, or upper lip. Patches often match on both sides of the face. This is sometimes called the mask of pregnancy
Numb or tingling hands, called carpal tunnel syndrome
Itching on the abdomen, palms, and soles of the feet. (Call your doctor if you have nausea, loss of appetite, vomiting, jaundice or fatigue combined with itching. These can be signs of a serious liver problem.)
Swelling of the ankles, fingers, and face. (If you notice any sudden or extreme swelling or if you gain a lot of weight really quickly, call your doctor right away. This could be a sign of preeclampsia.)
You're in the home stretch! Some of the same discomforts you had in your second trimester will continue. Plus, many women find breathing difficult and notice they have to go to the bathroom even more often. This is because the baby is getting bigger and it is putting more pressure on your organs. Don't worry, your baby is fine and these problems will lessen once you give birth.
Some new body changes you might notice in the third trimester include:
Shortness of breath
Heartburn
Swelling of the ankles, fingers, and face. (If you notice any sudden or extreme swelling or if you gain a lot of weight really quickly, call your doctor right away. This could be a sign of preeclampsia.)
Hemorrhoids
Tender breasts, which may leak a watery pre-milk called colostrum
Your belly button may stick out
Trouble sleeping
The baby "dropping," or moving lower in your abdomen
Contractions, which can be a sign of real or false labor
As you near your due date, your cervix becomes thinner and softer (called effacing). This is a normal, natural process that helps the birth canal (vagina) to open during the birthing process. Your doctor will check your progress with a vaginal exam as you near your due date.