The first trimester of pregnancy consists of weeks zero (the woman’s last menstrual period before fertilization occurred) to 12. During this time, a woman’s body starts adjusting to the developing embryo and begins to provide it with nourishment. As a result, a range of conditions may occur, most of which will disappear or lessen as the pregnancy progresses. The most common changes and symptoms that may occur during the first trimester include:

• Vaginal bleeding. Women may experience metrorrhagia (spotting) early in the pregnancy. In fact, a small amount of bleeding is often one of the fist signs of pregnancy. Light bleeding can occur when the fertilized egg implants in the wall of the uterus. This implantation bleeding typically takes place 6 to 12 days after conception. Women experiencing heavy vaginal bleeding, or any bleeding after the first trimester, should contact their obstetrician-gynecologist (ObGyn) immediately.
  
  
• Tender and enlarged breasts. An increase in breast size is another early sign of pregnancy. Breasts usually become swollen and enlarged during early pregnancy as the result of increased levels of the hormones progesterone and estrogen. The breasts may continue to grow throughout pregnancy. Pregnancy may also cause the breasts to become more sensitive and the nipples to darken. Women may benefit from wearing a more supportive bra.   
  
  
• Fatigue. Many women feel exhausted during the first trimester, regardless of how much sleep they are getting at night. This is the result of the body producing additional blood, releasing hormones and preparing to support the pregnancy. During this time women should rest as often as possible. Expectant mothers should try to get around eight hours of sleep a night and nap during the day. During the second trimester fatigue is generally replaced by feelings of increased energy, however, fatigue may return during the third trimester as the body carries the additional weight of the fetus.     
  
  
• Sleeplessness. Most women have trouble sleeping at some point during their pregnancy. This may include difficulty falling asleep, waking up during the night and difficulty returning to sleep once awakened. During the first trimester, the same hormones responsible for fatigue can cause lack of sleep at night. Sleeplessness may also appear in the later stages of pregnancy, as an enlarged abdomen makes finding a comfortable sleeping position difficult. Feelings of anxiety and stress may also contribute.
  
  
• Nausea and vomiting. It is common to experience nausea and vomiting during the first trimester, often 2 to 8 weeks after conception. Although this condition is often referred to as morning sickness, it can occur at any time of day. It is most likely the result of pregnancy hormones relaxing the stomach muscles. Women may benefit from eating frequent, small meals and drinking a lot of liquids. Nausea and vomiting usually disappear after the first trimester. Women who are vomiting a lot, or vomiting with pain or fever should immediately contact their ObGyn.
  
  
• Frequent urination. Many woman need to urinate more often during early pregnancy, usually beginning 6 to 8 weeks after conception. This is the result of the growing uterus pressing on the bladder. This pressure may also cause a woman to leak urine while sneezing, coughing or laughing (incontinence). Women experiencing pain or burning during urination, and those who notice pus or blood in the urine, should contact their ObGyn. This may indicate a urinary tract infection.
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• Cravings. Early in pregnancy, women may notice cravings for certain foods and increased hunger as the body begins to nourish the fetus.
  
  
• Constipation. Constipation may occur throughout pregnancy, but it is often worst in the first 13 to 14 weeks. A number of factors may contribute to constipation during pregnancy, including an increase in the hormone progesterone, which slows the digestive process. Additional pressure on the bowel and increased absorption of water by the colon may also be to blame. The increased amount of required iron found in prenatal vitamins may also contribute to constipation. Women may benefit from drinking eight to 10 glasses of water each day and increasing their fiber intake. Pregnant women should contact their ObGyn when they experience constipation leading to more than two days between bowel movements.
  
  
• Headaches. Many women experience headaches during pregnancy, especially during the first and third trimesters. Although their cause is uncertain, headaches during early pregnancy are most likely due to changes in hormone levels and increases in blood volume and circulation. Headaches experienced during the third trimester may be the result of carrying extra weight. Women experiencing a severe, persistent headache should contact their physician immediately, especially when dizziness, faintness, nausea or vomiting are also present.
  
  
• Mood swings. Pregnancy may cause extreme changes in mood in some women. Moods may range from excitement and happiness to irritation and depression. Mood swings may occur at any time during pregnancy but are common in the first trimester and in the weeks before delivery. They may be caused by hormonal changes, anxiety or discomfort. Women experiencing mood swings may benefit from eating fresh fruits and vegetables and whole grains, getting plenty of sleep, and using relaxation techniques such as meditation.
  
  
• Gas. The majority of pregnant women experience bloating and increased gas at some point during pregnancy. However, it is most common during the first trimester. This is due to hormones relaxing the muscles in the digestive tract. As a result, digestion is slowed and gas may build up. This often causes abdominal pain or discomfort, bloating, burping and passing gas. Women may be advised to avoid certain foods that may increase the symptoms.
  
  
• Acne. Elevated hormone levels can cause sebaceous glands in the skin to secrete more oil. As a result, acne may develop or worsen. However, some women experience an improvement in their acne.

There are many developments of the fetus during early pregnancy, and by the end of the first trimester the fetus is about 3 inches (76 millimeters) long and weighs about half an ounce (14 grams). The eyes and ears move into their correct positions and the kidneys begin to secrete urine into the bladder. Although an expectant mother is unable to feel the baby move at this point, it does begin to move in response to pushing on the abdomen.

Major developments during the first trimester include:

Week 0

• The woman has her last period before egg fertilization occurs

Week 2

• Fertilization occurs

Week 3

• The fertilized egg (zygote) implants in the wall of the uterus
  
  
• The amniotic sac (the membrane of fluid that surrounds the embryo throughout pregnancy) forms

Week 5

• The neural tube (the area that will become the brain and spinal cord) begins to develop

Week 6

• The heart and major blood vessels begin to develop
• A beating heart can be viewed during ultrasound

Week 7

• The beginning of arms and legs emerge

Week 9

• Bones and muscles are formed
• The skeleton is formed
• The face and neck develop
• The majority of organs are formed
• The fingers and toes are completely defined
• Brain waves are measurable

Week 10

• The kidneys begin to function
• The fetus is able to move and respond to touch (when prodded through the woman’s abdomen)