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Pregnancy begins when conception occurs.


The first sign of pregnancy is a missed menstrual period.

Swollen breasts and nausea with occasional vomiting may also be present. The swelling is caused by hormone changes similar to those a woman may have before her periods. The nausea and vomiting may be caused by hormonal changes produced by the cells of the placenta about 10 days after fertilization.

A woman who is pregnant may feel tired. Some women notice bloating of the abdomen early in a pregnancy.

Causes and Risk Factors

Pregnancy is caused when a sperm fertilizes an egg. It is a potential for sexually active women of childbearing age.

Certain factors may make the pregnancy high risk because they present risks to the health of the mother and baby and to having a normal pregnancy and delivery. These include:

  • Preexisting health conditions that make a pregnancy more complex or risky
  • Conditions that develop because of the new demands that a pregnancy makes on a woman's body, such as gestational diabetes
  • The nature of the mother's body, including height, weight, age or abnormalities in her uterus or pelvis
  • Abnormalities in the developing baby or its position inside its mother's body
  • A family history of twins, difficult pregnancies or genetic defects
  • A personal history of stillbirths, difficult pregnancies, babies with a genetic disorder or birth defect
  • Exposure to drugs, viruses or substances such as herpes simplex, viral hepatitis, mumps, rubella, varicella, syphilis, alcohol, cigarettes, lithium, streptomycin, tetracycline, thalidomide and warfarin.

Smoking and alcohol are two serious risk factors to a healthy pregnancy and delivery of a healthy baby. Drug addiction or substance abuse during pregnancy also represents serious risks to the health of the mother and baby.

All substances that a mother eats, drinks or breathes circulate through the baby's blood system as well as her own.

Ideally a woman should seek medical care before she becomes pregnant so that a doctor can:

  • Screen for disease
  • Advise her about the use of tobacco, drugs, alcohol or other substances
  • Check for physical conditions that may affect getting pregnant or having a healthy pregnancy and delivery
  • Educate her about maintaining the best health before conception and during pregnancy using diet, exercise and spacing of pregnancies
  • Inform her about the risks of exposure to cat litter (which can lead to toxoplasmosis), hot tubs, rubella, secondhand smoke and paint fumes


A doctor will take a woman's medical history and do a physical examination. If a woman has regular periods and is sexually active, missing a period by more than a week can usually be assumed as evidence of pregnancy.

A urine test for hormones that occur during pregnancy can usually allow a pregnancy to be confirmed several days after conception.

These signs are considered positive proof of pregnancy:

  • Fetal heart sounds as heard by a doctor or by Doppler ultrasound
  • Fetal movements felt or heard by a doctor
  • Identification of a sac inside the uterus and fetal heart motion
  • Delivery of a fetus

Pregnancies are usually measured in weeks, starting from the first day of the last menstrual period. Usually two weeks after missing a period, the patient is considered to be six weeks pregnant.

A number of changes will be observed in a pregnant woman's body at various stages of pregnancy, including:

  • In the first four weeks: the uterus is usually enlarged and irregularly softened; the cervix (the opening of the uterus) becomes softer and bluish or purple reflecting the increased blood supply to the uterus
  • At six weeks: The uterus can sometimes be easily flexed at the markedly softened isthmus. A cavity that is consistent with pregnancy can be detected within the uterus using ultrasound
  • At seven to eight weeks: the motion of the fetus' heart can be seen using ultrasound
  • At eight to 10 weeks: the fetus' heartbeat can be detected with a Doppler ultrasound device which can access the uterus abdominally
  • At 12 weeks: the uterus is larger than the pelvic area and rises into the abdomen. It can be felt above the pubic bone
  • At 16 weeks or later: a fetal skeleton can be identified on X-ray
  • At 18 to 20 weeks: a doctor can hear a fetal heart beat with a stethoscope
  • At 20 weeks: the upper point of the uterus is at the level of the woman's belly button
  • At 36 weeks: the upper point of the uterus is near the bottom of the breast bone

The baby is usually born 266 days from the time of conception or 280 days from the first day of the last menstrual period, if a woman's periods are regular. Baby due dates are estimates; it is not unusual for a baby to arrive two weeks before or two weeks after the due date estimated by the doctor.

Virtually every system of a woman's body is affected by pregnancy and returns to normal after delivery. Some of these changes include:

  • Increasing heart output
  • Pounding heart rate or abnormal heartbeats sometimes due to the pressure various organs put on the heart as the baby grows
  • Increased blood volume
  • A greater need for iron
  • Greater urine output. Depending on her position and what kind of pressure the baby is putting on her kidneys and bladder, a woman may feel a greater need to go to the bathroom when lying down and trying to sleep.
  • Changes in breathing due to hormone changes and to the pressure the growing baby is putting on the organs inside a woman's body. A pregnant woman may have a stuffy nose or ears because of blocks in the ears, nose and throat. The tone and quality of her voice may change. She may become breathless when she's active.
  • Constipation. This happens as the pregnancy proceeds because the enlarged uterus presses against the rectum and lower part of the colon. The muscles of the woman's digestive system tend to relax because of her changed hormones. She may have more heartburn and belching.
  • Greater risk of gallbladder problems
  • More active thyroid gland. This may cause a racing heart, sweating, emotional instability and an enlarged thyroid gland.
  • Rising adrenal hormone levels. This probably causes stretch marks and contributes to swelling.
  • Greater need for insulin due to how hormones affect the way glucose is used by the body. Women who are at risk of developing diabetes may do so during pregnancy.
  • Increased skin coloration. A blotchy, brownish pigment may appear over the forehead and cheekbones (mask of pregnancy or melasma). The areolae of the breasts may get darker. A dark line appears down the middle of the abdomen. The incidence of spider angiomas or tumor-like knots of blood vessels (usually only above the waist) and varicose veins (especially in the lower legs) also increases.


The process of becoming pregnant and giving birth can be divided into four stages:

  • Prenatal stage, which runs from conception to delivery
  • Labor, which is the process that runs from the last hours or days of pregnancy to delivery
  • Delivery, which is the process by which a baby is pushed from its mother's womb to ideally become an independently functioning newborn
  • After delivery, which is the time between the delivery of the baby and about four hours after the delivery of the placenta

© 2000-2022 The StayWell Company, LLC. All rights reserved. This information is not intended as a substitute for professional medical care. Always follow your healthcare professional's instructions.
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