Definition
Pregnancy is a period of up to 41 weeks in which a fetus develops inside a woman’s womb. NICHD conducts and supports research and training to help promote healthy pregnancies, with a focus on the important events that occur before, during, and after pregnancy.
Pregnancy is the term used to describe the period in which a fetus develops inside a woman’s womb or uterus.
Pregnancy usually lasts about 40 weeks, or just over 9 months, as measured from the last menstrual period to delivery. Healthcare providers refer to three segments of pregnancy, called trimesters.
Symptoms
The primary sign of pregnancy is missing a menstrual period or two or more consecutive periods, but many women experience other symptoms of pregnancy before they miss a period.
Missing a period does not always mean a woman is pregnant. Menstrual irregularities are common and can have a variety of causes, including taking birth control pills, conditions such as diabetes and polycystic ovary syndrome, eating disorders, and certain medications. Women who miss a period should see their healthcare provider to find out whether they are pregnant or whether they have another health problem.
Pregnancy symptoms vary from woman to woman. A woman may experience every common symptom, just a few, or none at all. Some signs of early pregnancy include:
- Slight bleeding. One study shows as many as 25% of pregnant women experience slight bleeding or spotting that is lighter in color than normal menstrual blood. This typically occurs at the time of implantation of the fertilized egg (about 6 to 12 days after conception) but is common in the first 12 weeks of pregnancy.
- Tender, swollen breasts or nipples. Women may notice this symptom as early as 1 to 2 weeks after conception. Hormonal changes can make the breasts sore or even tingly. The breasts feel fuller or heavier as well.
- Fatigue. Many women feel more tired early in pregnancy because their bodies are producing more of a hormone called progesterone, which helps maintain the pregnancy and encourages the growth of milk-producing glands in the breasts. In addition, during pregnancy the body pumps more blood to carry nutrients to the fetus. Pregnant women may notice fatigue as early as 1 week after conception.
- Headaches. The sudden rise of hormones may trigger headaches early in pregnancy.
- Nausea and/or vomiting. This symptom can start anywhere from 2 to 8 weeks after conception and can continue throughout pregnancy. Commonly referred to as “morning sickness,” it can actually occur at any time during the day.
- Food cravings or aversions. Sudden cravings or developing a dislike of favorite foods are both common throughout pregnancy. A food craving or aversion can last the entire pregnancy or vary throughout this period.
- Mood swings. Hormonal changes during pregnancy often cause sharp mood swings. These can occur as early as a few weeks after conception.
- Frequent urination. The need to empty the bladder more often is common throughout pregnancy. In the first few weeks of pregnancy, the body produces a hormone called human chorionic gonadotropin, which increases blood flow to the pelvic region, causing women to have to urinate more often.
Many of these symptoms can also be signs of other conditions, the result of changing birth control pills, or effects of stress, so they do not always mean that a woman is pregnant. Women should see their healthcare provider if they suspect they are pregnant.
Trimesters
First Trimester (Week 1 to Week 12)
The events that lead to pregnancy begin with conception, in which a sperm penetrates an egg. The fertilized egg (called a zygote) then travels through the woman’s fallopian tube to the uterus, where it implants itself in the uterine wall. The zygote is made up of a cluster of cells that later form the fetus and the placenta. The placenta connects the mother to the fetus and provides nutrients and oxygen to the fetus.
Second Trimester (Week 13 to Week 28)
- Between 18 and 20 weeks, the typical timing for ultrasound to look for birth defects, you can often find out the sex of your baby.
- At 20 weeks, a woman may begin to feel movement.
- At 24 weeks, footprints and fingerprints have formed and the fetus sleeps and wakes regularly.
- According to research from the NICHD Neonatal Research Network, the survival rate for babies born at 28 weeks was 92%, although those born at this time will likely still experience serious health complications, including respiratory and neurologic problems.
Third Trimester (Week 29 to Week 40)
- At 32 weeks, the bones are soft and yet almost fully formed, and the eyes can open and close.
- Infants born before 37 weeks are considered preterm. These children are at increased risk for problems such as developmental delays, vision and hearing problems, and cerebral palsy. Infants born between 34 and 36 weeks of pregnancy are considered to be “late preterm.”
- Infants born in the 37th and 38th weeks of pregnancy—previously considered term—are now considered “early term.” These infants face more health risks than infants who are born at 39 weeks or later, which is now considered full-term.
- Infants born at 39 or 40 weeks of pregnancy are considered full-term. Full-term infants have better health outcomes than infants born earlier or, in some cases, later than this period. Therefore, if there is no medical reason to deliver earlier, it is best to deliver at or after 39 weeks to give the infant’s lungs, brain, and liver time to fully develop.
- Infants born at 41 weeks through 41 weeks and 6 days are considered late term.
- Infants who are born at 42 weeks and beyond are considered post-term.
Prenatal Care
Having a healthy pregnancy is one of the best ways to promote a healthy birth. Getting early and regular prenatal care improves the chances of a healthy pregnancy. This care can begin even before pregnancy with a pre-pregnancy care visit to a health care provider.
Pre-Pregnancy Care
Complications
Getting early and regular prenatal care can help decrease the risk for problems by enabling healthcare providers to diagnose, treat, or manage conditions before they become serious. Prenatal care can also help identify mental health concerns related to pregnancy, such as anxiety and depression.
Some common complications of pregnancy include, but are not limited to, the following.
High Blood Pressure
Labor
Labor is the process by which the fetus and the placenta leave the uterus. Delivery can occur in two ways, vaginally (through the birth canal) or by a cesarean (surgical) delivery.
A cesarean delivery is a surgical procedure in which a fetus is delivered through an incision in the mother’s abdomen and uterus.