Free Guide to Understanding Gestational Age in Weeks

What Gestational Age Means and Why It Matters

Gestational age is the number of weeks that have passed since the first day of a pregnant person's last menstrual period (LMP). Healthcare providers use this measurement to track pregnancy progress, monitor fetal development, and plan care throughout the nine months of pregnancy. Understanding gestational age helps you know what to expect during each stage and when certain milestones occur.

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The term "gestational" refers to the period of gestation—the time during which a baby develops in the womb. While the actual fertilization and conception may occur around two weeks after the LMP, healthcare providers count from the first day of the last menstrual period because this date is usually easier for people to remember than the exact moment of conception. This means that when you are told you are "4 weeks pregnant," approximately two weeks have passed since conception.

Gestational age differs from fetal age, which measures time from conception. For example, at 4 weeks of gestational age, the fetal age is approximately 2 weeks. Healthcare providers almost always refer to gestational age in medical settings because it provides a standardized measurement that works across different pregnancies and populations.

The pregnancy timeline is divided into three periods called trimesters. The first trimester spans weeks 1 through 13, the second trimester covers weeks 14 through 27, and the third trimester includes weeks 28 through 40. Each trimester brings different changes in the body and different stages of fetal development. Knowing your gestational age helps you understand where you are in this progression and what physical and emotional changes you might experience.

Practical takeaway: Write down the first day of your last menstrual period and share this date with your healthcare provider at your first prenatal visit. This single piece of information forms the foundation for calculating your gestational age and estimated due date throughout your pregnancy.

How Gestational Age Is Calculated

The standard method for calculating gestational age starts with the first day of the last menstrual period. If you remember this date, your healthcare provider can count forward by weeks and days to determine your current gestational age. A typical pregnancy lasts 40 weeks from the LMP, though babies born between 37 and 42 weeks are generally considered full-term.

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If you do not remember the exact date of your last menstrual period, ultrasound imaging can estimate gestational age. In the first trimester (before 13 weeks and 6 days), ultrasound measurements are most accurate, typically within 3 to 5 days of the actual age. During the second trimester (14 to 27 weeks), the margin of error increases to about 10 to 14 days. In the third trimester (28 weeks and beyond), ultrasound estimates become less precise, with a margin of error of about 2 to 3 weeks.

Ultrasound technicians measure specific features to estimate gestational age depending on the trimester. In early pregnancy, they measure the gestational sac and the fetal pole. Later in the first trimester, they measure the crown-rump length (the distance from the top of the fetal head to the bottom of the buttocks). In the second trimester, measurements include head circumference, femur length (thighbone), and abdominal circumference. These physical measurements correspond to expected growth patterns for different gestational ages.

Some healthcare providers also consider other signs to confirm gestational age, including when you first hear the fetal heartbeat, when you first feel fetal movement (quickening), and the size of the uterus. For people with irregular menstrual cycles, medical history of hormonal birth control, or other factors that make the LMP less reliable, ultrasound confirmation becomes especially valuable.

Practical takeaway: Bring all relevant dates to your first prenatal appointment, including when your last menstrual period began, any unusual menstrual patterns, and when you first noticed pregnancy symptoms. The more information you provide, the more accurate your gestational age calculation will be.

Gestational Age Throughout the Three Trimesters

The first trimester (weeks 1-13) marks the beginning of pregnancy and the most critical period of organ and tissue formation. During these early weeks, the fertilized egg travels to the uterus and implants in the uterine lining. By the end of the first week, cell division creates a structure called a blastocyst. By week 4, the embryo is about the size of a poppy seed, and the neural tube (which becomes the brain and spinal cord) begins to form. By week 8, all major organs have begun to develop, and the embryo is now called a fetus. By the end of the first trimester at week 13, the fetus measures about 3 inches long and weighs about half an ounce.

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Physical changes during the first trimester can include missed periods, breast tenderness, nausea (sometimes called morning sickness), fatigue, and frequent urination. Many people experience food cravings or aversions. Mood changes are common due to hormonal shifts. The first trimester also carries the highest risk for miscarriage, though risk decreases significantly after week 12 when the risk drops to about 1 percent.

The second trimester (weeks 14-27) is often called the "honeymoon period" of pregnancy because many pregnancy discomforts ease. By the start of this trimester, most organs have formed, and the fetus now focuses on growth and development. By week 20, the fetus measures about 6 to 7 inches and weighs approximately 5 ounces. By week 27, the fetus is about 14 inches long and weighs around 2 pounds. During these weeks, facial features become more defined, and the fetus develops the ability to hear. Most people feel the baby's movements, called quickening, sometime between weeks 16 and 25.

Physical changes in the second trimester include a visibly growing belly, continued weight gain, possible darkening of the skin on the face (melasma), a dark line down the middle of the abdomen (linea nigra), and possible stretch marks. Energy levels often increase after the fatigue of the first trimester. By the end of the second trimester, the fetus has survived the critical period of organ formation and has developed a reasonable chance of survival if born with medical support.

The third trimester (weeks 28-40) focuses on rapid fetal growth and preparation for life outside the womb. Between weeks 28 and 36, the fetus may move into a head-down position in preparation for birth. By week 32, the fetus weighs approximately 4 pounds, and by week 36, about 6 pounds. By week 40, most fetuses weigh between 7 and 8 pounds. During the final weeks, the fetus develops the ability to regulate body temperature and continued maturation of the lungs, digestive system, and immune system.

Physical sensations in the third trimester include increased back pain and pelvic pressure, difficulty finding comfortable sleeping positions, increased swelling in the feet and ankles, and possible braxton-hicks contractions (practice contractions). Many people experience mood changes as the reality of approaching birth becomes clear. Around week 36, the healthcare provider may discuss warning signs that warrant contacting them immediately, such as vaginal bleeding, severe abdominal pain, or loss of fetal movement.

Practical takeaway: Create a simple timeline marking the start and end of each trimester. Note when you expect to feel movement, when routine screenings typically occur, and when certain physical changes may begin. This helps you understand what to anticipate and recognize normal progression.

Common Milestones at Different Gestational Ages

Gestational age helps predict when certain milestones will occur. At week 5, the fetal heartbeat becomes visible on ultrasound, typically detected as a flickering motion. This moment means the pregnancy is developing as expected and helps confirm the date of conception. Around week 8 to 12, a dating ultrasound (also called the nuchal translucency scan) is often performed to measure the fetus and confirm or adjust the due date. Around week 11 to 13, screening for chromosomal abnormalities may be offered through blood tests combined with ultrasound measurements.

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During the second trimester, around week 18 to 22, the anatomy scan (also called the fetal survey