First Trimester of Pregnancy

Medically Reviewed by Traci C. Johnson, MD on March 22, 2023
8 min read

The first trimester is the earliest phase of pregnancy. It starts on the first day of your last period -- before you’re even actually pregnant -- and lasts until the end of the 13th week. It’s a time of great anticipation and of rapid changes for both you and your baby. Knowing what to expect will help you get ready for the months ahead.

Pregnancy is different for every woman. Some women glow with good health during those first 3 months; others feel absolutely miserable. Here are some of the changes you might notice, what they mean, and which signs warrant a call to your doctor.

Bleeding. About 25% of pregnant women have slight bleeding during their first trimester. Early in the pregnancy, light spotting may be a sign that the fertilized embryo has implanted in your uterus. But if you have severe bleeding, cramping, or sharp pain in your belly, call the doctor. These could be signs of a miscarriage or ectopic pregnancy (a pregnancy in which the embryo implants outside of the uterus).

Breast tenderness. Sore breasts are one of the earliest signs of pregnancy. They're triggered by hormonal changes, which are getting your milk ducts ready to feed your baby. Your breasts will probably be sore throughout the first trimester. Going up a bra size (or more) and wearing a support bra can make you feel more comfortable. You probably won’t go back to your regular bra size until after your baby is finished nursing.

Constipation. During pregnancy, high levels of the hormone progesterone slow down the muscle contractions that normally move food through your system. Add to that the extra iron you're getting from your prenatal vitamin, and the result is uncomfortable constipation and gas that can keep you feeling bloated throughout your pregnancy. Eat more fiber and drink extra fluids to keep things moving more smoothly. Physical activity can also help.

If your constipation is really bothering you, talk to your doctor about what mild laxative or stool softeners are safe to use during pregnancy.

Discharge. It's normal to see a thin, milky white discharge (called leukorrhea) early in your pregnancy. You can wear a panty liner if it makes you feel more comfortable, but don't use a tampon because it could put germs into your vagina. If the discharge smells really bad, if it’s green or yellow, or if there's a lot of clear discharge, call the doctor.

Fatigue. Your body is working hard to support a growing baby. That means you’ll get tired more easily than usual. Take naps or rest when you need to during the day. Make sure you're getting enough iron. Too little can lead to anemia, which can make you even more tired.

Food likes and dislikes. Although you may not want a bowl of mint chip ice cream topped with dill pickles, as the old stereotype goes, your tastes can change while you're pregnant. More than 60% of pregnant women have food cravings. More than half have foods they really don’t like. Giving in to cravings from time to time is OK, so long as you eat healthy, low-calorie foods most of the time. The exception is pica -- a craving for nonfoods like clay, dirt, and laundry starch, which can be dangerous for you and your baby. If you experience this kind of craving, report it to your doctor right away.

Peeing a lot. Your baby is still pretty small, but your uterus is growing and it's putting pressure on your bladder. As a result, you may feel like you have to go to the bathroom all the time. Don't stop drinking fluids -- your body needs them -- but do cut down on caffeine (which stimulates your bladder), especially before bedtime. When nature calls, answer it as soon as you can. Don't hold it in.

Heartburn. During pregnancy, your body produces more of the hormone progesterone. It relaxes smooth muscles, like the ring of muscle in your lower esophagus, the tube that connects your mouth and stomach. These muscles normally keep food and acids down in your stomach. When they loosen up, you can get acid reflux, otherwise known as heartburn. To avoid the burn:

  • Eat a few small meals throughout the day.

  • Don't lie down right after you eat.

  • Avoid greasy, spicy, and acidic foods (like citrus fruits).

  • Try raising your pillows when you sleep.

Mood swings. Increased fatigue and changing hormones can put you on an emotional roller coaster that takes you from joyous to miserable, or from hopeful to terrified in a matter of seconds. It's OK to cry, but if you feel overwhelmed, try to find an understanding ear. You can talk to your partner, a friend, a family member, or even a professional.

Morning sickness. Nausea is one of the most common pregnancy symptoms. Up to 85% of pregnant women have it. It results from hormone changes in your body and it can last through the entire first trimester. For some pregnant women, nausea is mild. Others can't start their day without vomiting. Nausea is usually worse in the morning (hence the name, "morning sickness"). To calm your nausea, try eating small, bland, or high-protein snacks (crackers, meat, or cheese) and sipping water, clear fruit juice (apple juice), or ginger ale. You may want to even do this before you get out of bed. Avoid any foods that make you sick to your stomach. Nausea itself isn't anything to worry about, but if it’s severe or just won’t go away, it can affect the amount of nutrition your baby gets. Call your doctor if you can't stop throwing up or can't keep down any food.

Weight gain. Pregnancy is one of the few times in a woman's life when weight gain is considered a good thing, but don't overdo it. During the first trimester, you should gain about 3-6 pounds (your doctor may suggest you adjust your weight gain up or down if you started your pregnancy underweight or overweight). Although you're carrying an extra person, you really aren’t eating for two. You only need about an extra 150 calories a day during the first trimester. Get those calories the healthy way, by adding extra fruits and vegetables, milk, whole-grain bread, and lean meat to your diet.


During the first 13 weeks, your baby changes from a fertilized egg into a fully-formed fetus. All the major organs and systems are taking shape. That means your baby could be harmed if you use street drugs, have an illness, or get exposed to radiation. Here’s what’s happening:

  • The fertilized egg becomes a cluster of rapidly dividing cells that implants in your uterus. The placenta, umbilical cord, and amniotic sac all start to grow.

  • Your baby’s nervous system changes from an open neural tube to a brain and spinal cord. Nerves and muscles start working together. Your baby can move on its own, but it’s too soon for you to feel it.

  • The heart takes shape and begins to beat. You can hear it on ultrasound as early as week 6. It beats 120 to 160 times per minute. Red blood cells are forming.
  • Your baby develops a digestive system, including intestines and kidneys.
  • They have lungs and other major organs, but they’re not fully developed.
  • A soft skeleton is starting to grow.
  • Your baby starts to look like a baby, with arms, legs, fingers, and toes. Their face gets eyes, ears, a nose, and mouth. A tongue and tooth buds grow. Eyelids cover your baby’s eyes, and by the end of the trimester, they even have fingernails.

  • Genitals start to grow, but it’s too early to tell by ultrasound whether you’re having a girl or a boy.

By the end of the first trimester, your baby will be about 2 ½-3 inches long.


Having a baby is one of the most joyous times in many women's lives. From thinking about the day you'll bring your little one home, to picking a name and nursery colors, the excitement is intense. But you also need to take some practical steps during the first trimester, including:

  • Choose a doctor. Do you want an obstetrician or a midwife? Get referrals and find out what your health insurance covers.

  • Schedule a prenatal visit as soon as you know you’re pregnant. You’ll cover a lot of ground in the first appointment. The doctor will take a full medical history and talk to you about your lifestyle and health habits. They’ll figure out your due date. You’ll also have blood and urine tests and possibly a pelvic exam.

  • Continue with prenatal visits every 4 weeks. The doctor will check your weight and blood pressure, test your urine and listen to your baby’s heartbeat.

  • Learn what other tests and screenings you may need, such as tests to look for genetic problems with your baby.

  • Start taking a prenatal vitamin with at least 400 micrograms of folic acid to help your baby’s brain and spinal cord grow properly.

  • Ask your doctor what prescription and over-the-counter medicines you can still safely take.

  • Take a look at your diet and make any changes you need to make sure you and your baby get the right nutrition. Drink plenty of water.

  • Break bad habits like smoking and illegal drug use. Cut out alcohol and cut down on caffeine.

  • Keep up your workout routine, but listen to your body. You may need to change what kind of exercise you do, or ease up a little.

  • Research what it costs to have a baby and start making changes. Will you have to pay for child care? Will you cut back on work? Draw up a new budget that reflects the new addition.

  • Decide when and how you’ll share your news. You may want to wait until you’ve heard the baby’s heartbeat, or made it safely past the first trimester. It’s also smart to find out your company’s policies on maternity leave and learn your rights before you tell your boss.

Any of these symptoms could be a sign that something is seriously wrong with your pregnancy. Don't wait for your prenatal visit to talk about it. Call your doctor right away if you experience: