The first trimester of pregnancy is the earliest phase, lasting from the first day of your last menstrual period (LMP) until the end of the 13th week. It’s a time of great anticipation and of rapid changes for both you and your baby.

Baby development

During the first trimester, your baby changes from a fertilized egg into a fully-formed fetus. All the major organs and systems are taking shape. By the end of the 13th week, your baby will be about 3 inches long and weigh about an ounce.

Here’s a brief overview of what happens during each week of the first trimester:

• Weeks 1-4: The fertilized egg travels down the fallopian tube and implants in the lining of the uterus. This is called implantation. The placenta, umbilical cord, and amniotic sac begin to form.
• Weeks 5-8: The baby’s heart begins to beat. The brain, eyes, ears, nose, and mouth start to develop. Arms and legs begin to form as buds.
• Weeks 9-12: All the major organs and systems are now present. The baby’s fingers and toes are starting to separate. The baby is now called a fetus.

Your body changes

You may start to notice some physical changes during the first trimester, including:

• Tender, swollen breasts: This is caused by hormonal changes.
• Nausea with or without vomiting: This is a common symptom of pregnancy, also known as morning sickness. It can occur at any time of day or night.
• Increased urination: You may find yourself urinating more often than usual. This is because your blood volume is increasing and your kidneys are working harder to filter waste.
• Fatigue: During early pregnancy, levels of the hormone progesterone soar, which can make you feel tired.
• Food cravings and aversions: You may develop cravings for certain foods or aversions to others. This is thought to be caused by hormonal changes.
• Heartburn: This is a common symptom of pregnancy, caused by the stomach acid backing up into the esophagus.
• Constipation: This is another common symptom of pregnancy, caused by the hormones relaxing the muscles in the digestive system.

Prenatal care

It’s important to start prenatal care as soon as possible after you find out you’re pregnant. Your doctor will monitor your health and the development of your baby. They will also provide you with information on how to have a healthy pregnancy.

Here are some tips for having a healthy first trimester:

• Eat a healthy diet.
• Get regular exercise.
• Avoid smoking and drinking alcohol.
• Manage stress.
• Get enough sleep.

If you have any concerns during the first trimester, be sure to talk to your doctor.