For a pregnant woman, feeling a new life developing inside her body is an amazing experience, even though she may not always feel her best at some points along the way.
Pregnancy can be different from woman to woman, and even for the same mother from one pregnancy to the next. Some symptoms of pregnancy last for several weeks or months, while other discomforts are temporary or don't affect all women.
"Pregnancy is a long, 10-month journey," said Dr. Draion Burch, an obstetrician and gynecologist at Magee-Womens Hospital at the University of Pittsburgh Medical Center.
A normal pregnancy usually lasts about 40 weeks, counting from the first day of a woman's last menstrual period, which is about two weeks before conception actually occurs.
Pregnancy is divided into three trimesters. Each of these periods lasts between 12 and 13 weeks.
During each trimester, changes take place in a pregnant woman's body as well as in the developing fetus, and a summary of these changes will be described below.
Conception and implantation
About two weeks after a woman has her period, she ovulates and her ovaries release one mature egg. The egg can be fertilized for 12 to 24 hours after it's released as it travels down the fallopian tube toward the uterus.
If an egg meets up with a sperm cell that has made its way into the fallopian tube, it combines into one cell, a process that's known as fertilization or conception.
At fertilization, the sex of the fetus is already determined, depending on whether the egg receives an X or Y chromosome from a sperm cell. If the egg receives an X chromosome, the baby will be a girl; a Y chromosome means the baby will be a boy.
According to the Cleveland Clinic, it takes about three to four days for the fertilized egg (or embryo) to move to the lining of the uterus, where it attaches or implants to the uterine wall. Once the embryo is implanted, the cells start to grow, eventually becoming the fetus and the placenta, which is tissue that can transport oxygen, nutrients and hormones from the mother's blood to the developing fetus throughout pregnancy.
First trimester (weeks 1-12) changes in the mother's body
A woman will experience a lot of symptoms during her first trimester as she adjusts to the hormonal changes of pregnancy. In the early weeks, the pregnancy may not be showing much on the outside of her body, but inside many changes are taking place.
For example, human chorionic gonadotrophin (hCG) is a hormone that will be present in a woman's blood from the time conception occurs. Levels of hCG can be detected in a woman's urine about a week after she has a missed period, and it is why a woman will have a positive result on a home pregnancy test.
Other hormonal changes can contribute to pregnancy symptoms: Rising levels of estrogen and hCG may be responsible for the waves of nausea and vomiting known as morning sickness that a woman typically feels during her first few months of pregnancy. Despite its name, morning sickness can occur any time of day.
A woman will also feel more tired than usual during the first trimester, a symptom that's linked with rising levels of the hormone progesterone, which increases sleepiness. She may also need to urinate more frequently as her uterus grows and presses on her bladder.
Early in pregnancy, a woman's breasts will feel more tender and swollen, another side effect of rising levels of pregnancy hormones. Her areolas, the skin around each nipple, will darken and enlarge.
A pregnant woman's digestive system may slow down to increase the absorption of beneficial nutrients. But reduced mobility of the digestive system might also trigger such common complaints as heartburn, constipation, bloating and gas, according to the Office on Women's Health (OWH).
Many parts of the body will work harder during pregnancy, including a woman's heart. Her heartbeat will increase to pump more blood to the uterus, which will supply it to the fetus.
As more blood circulates to a woman's face, it will give her skin a rosier complexion, described as a "pregnancy glow."
Besides the physical changes in a woman's body, she may also experience emotional highs and lows in the early months of her pregnancy and throughout it. These emotions may range from weepiness, mood swings and forgetfulness to fear, anxiety and excitement.
First trimester development of embryo/fetus
A developing baby is called an embryo from the moment conception takes place until the eighth week of pregnancy.
During the first month of pregnancy the heart and lungs begin to develop, and the arms, legs, brain, spinal cord and nerves begin to form, according to the American College of Obstetricians and Gynecologists (ACOG).
The embryo will be about the size of a pea around one month into a pregnancy, Burch said. Around the second month of pregnancy, the embryo has grown to the size of a kidney bean, he explained. In addition, the ankles, wrists, fingers and eyelids form, bones appear, and the genitals and inner ear begin to develop.
After the eighth week of pregnancy and until birth occurs, a developing baby is called a fetus.
By the end of the second month, eight to 10 of the fetus' main organs will have formed, Burch said. At this stage of pregnancy, he stressed, it's extremely important that pregnant women do not take harmful medications, such as illegal drugs. The first trimester is also the period when most miscarriages and birth defects occur.
During the third month of pregnancy, bones and muscles begin to grow, buds for future teeth appear, and fingers and toes grow. The intestines begin to form and the skin is almost transparent.
Second trimester (weeks 13-27) changes in the mother's body
By the second trimester, some of the unpleasant effects of early pregnancy may lessen or disappear as a woman's body adjusts to its changing hormone levels. Sleeping may get easier and energy levels may increase.
Nausea and vomiting usually get better and go away, Burch told Live Science. But other symptoms may crop up as the fetus continues its growth and development.
Women feel more pelvic pressure, Burch said, adding that the pelvis feels heavy like something is weighing it down.