Pregnancy is divided into three stages called trimesters, and each one lasts about 13 to 14 weeks. The three stages of pregnancy are the first trimester (week 1 through week 13), the second trimester (week 14 through week 27), and the third trimester (week 28 through birth). Each stage brings specific physical changes for the mother and specific developmental milestones for the baby.
What Are The Three Stages Of Pregnancy?
The three stages of pregnancy are the first, second, and third trimesters. Healthcare providers use these divisions to track fetal development and to schedule prenatal tests. Each trimester has a distinct set of changes, though the exact timing of symptoms varies from person to person.
The first trimester starts on the first day of your last menstrual period, not the day of conception. This means you are considered about two weeks pregnant at the moment of fertilization. The second trimester is often described as the most comfortable period for many women. The third trimester brings the baby’s final growth and prepares the body for labor.
First Trimester: Weeks 1 to 13
The first trimester is the period of rapid development. The fertilized egg becomes a blastocyst, implants in the uterine lining, and then develops into an embryo. By the end of week 8, the embryo is officially called a fetus.
During these weeks, all major organs begin to form. The neural tube, which becomes the brain and spinal cord, develops early. The heart starts beating around week 6, often detectable on an ultrasound. Arms, legs, fingers, and toes form. By the end of the first trimester, the fetus is about 3 inches long and weighs about 1 ounce.
Hormone levels rise sharply during this trimester, especially human chorionic gonadotropin (hCG) and progesterone. These hormones are necessary to maintain the pregnancy, but they also cause many of the classic early symptoms.
Common First Trimester Symptoms
- Nausea with or without vomiting, often called morning sickness
- Fatigue and exhaustion
- Tender, swollen breasts
- Frequent urination
- Food aversions or cravings
- Mood swings
Morning sickness is a common term, but the nausea can occur at any time of day. Some research suggests it is linked to rising hCG levels. Most women notice significant improvement by the end of the first trimester.
This is also the trimester with the highest risk of miscarriage. Most miscarriages occur before week 12, often because of chromosomal abnormalities in the embryo. This is not caused by anything the mother did or did not do.
Second Trimester: Weeks 14 to 27
The second trimester is often the easiest stage for many women. Nausea usually decreases or disappears. Energy levels often return. The risk of miscarriage drops significantly after the first trimester.
The baby grows rapidly during these weeks. Muscles strengthen, and bones harden. The fetus begins to move, and most women feel these movements, called quickening, between weeks 18 and 22. The skin is covered in a fine, soft hair called lanugo, which helps protect the skin.
By week 20, an ultrasound can usually determine the baby’s sex. The fetus can hear sounds from outside the womb. The eyes open around week 27, and the brain continues to develop complex structures.
Physical Changes in the Mother
The uterus expands significantly during the second trimester. This growth shifts the center of gravity, which can cause lower back pain. The skin on the abdomen stretches, and many women develop a dark line called the linea nigra.
Some women experience round ligament pain, a sharp or jabbing sensation in the lower abdomen or groin. This happens as the ligaments that support the uterus stretch. It is usually brief and harmless.
Blood volume increases by about 40 to 50 percent during pregnancy. This can cause nasal congestion, nosebleeds, and bleeding gums. These are common pregnancy effects, not signs of illness.
Third Trimester: Weeks 28 to Birth
The third trimester is the final stretch. The baby gains the most weight during these weeks. By week 32, most babies are in a head-down position in preparation for birth.
The baby’s lungs mature significantly during this trimester. The brain grows rapidly, and the nervous system develops further. By week 37, the baby is considered early term and is generally developed enough for delivery.
Most of the baby’s weight gain happens in the third trimester. At birth, the average full-term baby weighs between 6 and 9 pounds and measures about 18 to 21 inches long.
Common Third Trimester Symptoms
- Shortness of breath as the uterus presses on the diaphragm
- Frequent urination as the baby drops lower into the pelvis
- Swelling in the ankles, feet, and hands
- Braxton Hicks contractions, which are practice contractions
- Difficulty sleeping
- Heartburn and indigestion
Braxton Hicks contractions are irregular and do not get closer together. True labor contractions are regular, increase in intensity, and do not stop with movement or rest. If contractions become regular, contact your healthcare provider.
Sudden or severe swelling in the face or hands, severe headaches, or vision changes can be signs of preeclampsia. These symptoms require immediate medical attention.
How the Trimesters Are Calculated
Pregnancy is measured from the first day of your last menstrual period. This means the first two weeks of the 40-week count happen before conception. A full-term pregnancy is 40 weeks, which is about 280 days.
The due date is an estimate. Only about 5 percent of babies are born on their exact due date. Most births occur between 38 and 42 weeks.
Healthcare providers may adjust the due date based on an early ultrasound. Ultrasound measurements of the fetus are most accurate in the first trimester. Later in pregnancy, fetal size varies too much for accurate dating.
Prenatal Care Across the Trimesters
Prenatal visits happen more often as pregnancy progresses. In the first trimester, most women visit their provider monthly. The first visit typically includes blood tests, urine tests, and a full physical exam.
Between weeks 24 and 28, most providers screen for gestational diabetes. This involves drinking a sugary solution and having blood drawn. This test is standard practice because gestational diabetes often has no symptoms.
In the third trimester, visits become more frequent. Most providers schedule appointments every two weeks from week 28 to week 36, then weekly until birth. These visits track blood pressure, fetal growth, and the baby’s position.
Vaccination during pregnancy is an important part of prenatal care. The Tdap vaccine is recommended during each pregnancy, ideally between weeks 27 and 36. This vaccine passes protective antibodies to the baby, who is too young to be vaccinated against whooping cough directly.
Key Developmental Milestones by Trimester
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First | 1–13 | Major organs form; heart begins beating; neural tube closes |
| Second | 14–27 | Fetal movements felt; sex visible on ultrasound; hearing develops |
| Third | 28–birth | Lungs mature; rapid weight gain; baby positions for birth |
When to Call Your Healthcare Provider
Some symptoms require immediate medical attention at any stage of pregnancy. Vaginal bleeding, severe abdominal pain, and fluid leaking from the vagina should always be evaluated. A significant decrease in fetal movement in the third trimester also warrants a call to your provider.
Fever above 100.4°F, severe headache, vision changes, or sudden swelling in the face or hands can indicate infection or preeclampsia. Do not wait for these symptoms to resolve on their own. Contact your provider or go to an emergency room.
Mild cramping and light spotting can be normal, but only a healthcare provider can determine whether symptoms are concerning. When in doubt, call. That is what prenatal care is for.
Frequently Asked Questions
How long does each trimester last?
Each trimester lasts about 13 to 14 weeks. The first trimester is weeks 1 through 13, the second is weeks 14 through 27, and the third is weeks 28 through birth.
When does morning sickness usually start and end?
Morning sickness typically starts around week 6 and improves by the end of the first trimester. Some women experience nausea into the second trimester, and a small number have it throughout pregnancy.
When can I feel the baby move?
Most women feel fetal movement between weeks 18 and 22. First-time mothers often notice it later, while women who have been pregnant before may feel it earlier.
What is the safest week to give birth?
A baby born between 39 and 40 weeks is considered full term, and this is the safest window for delivery. Babies born before 37 weeks are preterm and may need extra medical support.
