Prenatal Development
Each trimester is marked by a baby's growth and by body changes in the mother.

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Prenatal Development

Physicians and expectant moms alike tend to mark milestones in terms of three trimesters. Each trimester is marked by a baby’s growth and by body changes in the mother.

What Are the Major Milestones of the First Trimester?

The first trimester of pregnancy is weeks one through 12. This is a time of great growth and adjustment as the woman’s body experiences greater hormone production to support the baby. Symptoms women experience during the first trimester include:

  • constipation
  • feeling more fatigued than usual
  • frequent need to urinate
  • headache
  • heartburn
  • mood swings
  • nausea or vomiting, particularly upon waking—this is commonly known as morning sickness
  • tender and/or swollen breasts
  • unusual food cravings
  • weight gain or even weight loss

Addressing Discomforts

Increasing hormones in your body may make you feel nauseous. Always check with your physician before taking any medications, as they could affect your baby. While not all women experience morning sickness, eating some bland crackers and drinking plenty of fluids upon waking can help. If possible, try to get more sleep to reduce feelings of fatigue.

Nutritional Needs

Good nutrition during the first three months of pregnancy is vital for your baby’s neural development. This includes getting enough protein, iron, calcium, and folic acid, which is essential for healthy nervous system growth. It is typical to gain between two and four pounds during your first trimester (NIH). However, this amount varies. Women who were underweight prior to becoming pregnant should gain more than women who were overweight, according to the NIH.

Your Baby’s Growth

As early as one month into your pregnancy, your baby will already be forming a heart, brain, and spinal cord. After about eight weeks, your baby’s heart will beat at a regular rhythm. At the end of your first trimester, your baby is about three inches long and weighs an ounce.

What Are the Major Milestones of the Second Trimester?

The second trimester lasts from week 13 to week 28 in your pregnancy. This trimester typically represents the subsiding of nausea symptoms, due to more regulated hormones. However, your baby will experience significant growth during this trimester, and this can present a new set of symptoms. These include:

  • areas of darkened skin, particularly on the face
  • body aches, including the back, groin, stomach, and legs
  • increased itching, particularly on the stomach, palms, and soles of the feet
  • increased swelling, particularly in the ankles, fingers, and face

If you experience extreme fluctuations in symptoms, such as gaining several pounds quickly, yellowing skin, or nausea that does not subside with time, report these symptoms to your doctor immediately.

Nutritional Needs

Follow your healthcare team’s instructions, which may differ from general guidelines. Most women need to eat at least 300 more calories per day than they did before pregnancy. Choose healthy, whole foods instead of empty-calorie foods that are high in sugar and salt, such as candy and fast foods. You can expect to gain between three and four pounds during your second trimester, according to the NIH.

Your Baby’s Growth

The second trimester is when you may begin to feel your baby moving in the womb. This typically occurs at about 20 weeks. Your baby develops eyebrows, fingernails, toenails, footprints, and fingerprints during the second trimester. He or she can also make facial expressions. The baby will wake and sleep regularly and will reach about 1.5 pounds by the end of this trimester.

What Are the Major Milestones of the Third Trimester?

The third trimester lasts from week 29 to the baby’s birth. A baby grows quickly during this time and will become increasingly active. Many women continue to experience symptoms from the second trimester, as well as other symptoms, including:

  • acid reflux or heartburn
  • a belly button that starts to stick out 
  • difficulty sleeping
  • swelling, especially in the ankles, fingers, and/or face
  • tender breasts that may leak a watery substance called colostrum
  • trouble breathing

Your baby will move lower in your belly to prepare for birth. You also may experience some premature, false contractions known as Braxton-Hicks contractions. One way to tell the difference between false contractions and true labor is that false contractions typically resolve with activity, such as walking. If you experience more than four contractions in an hour, contact your medical provider.

Nutritional Needs

Most women gain three or four more pounds during their third trimester. This weight includes the baby’s weight, increased blood volume, amniotic fluid, fat stores, and uterus growth.

Your Baby’s Growth

At 32 weeks of age, your baby’s bones are fully formed but not fully hardened. He or she gains roughly 0.5 pounds each week beginning at 32 weeks. Your baby is gaining more body fat, meaning it has less room to move around. For this reason, you will feel your baby move, but the movements may not feel as forceful. Your baby is considered full-term when the pregnancy is at 37 weeks.

This information is a summary. Always seek medical attention if you are concerned you may be experiencing a medical emergency.

Written by: Rachel Nall, RN, BSN
Edited by:
Medically Reviewed by: George Krucik, MD, MBA
Published: Jun 12, 2014
Published By: Healthline Networks, Inc.
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