What Is Gestational Age?

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Gestation is a term that describes the time between conception and birth, during which a baby grows and develops in the pregnant parent's uterus. Gestational age refers to how far along the pregnancy is and is generally expressed as a combination of weeks and days.

Gestational age helps to estimate a possible due date, inform obstetrical care and testing, and evaluate the baby's health at birth. It serves as a guideline to determine if the baby is growing as expected and when to perform certain prenatal screenings. Babies born smaller or bigger than expected for their gestational age may need to be monitored more closely.

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Determining Gestational Age

Parents do not always know when conception occurred. Healthcare providers can determine weeks of gestation by looking at the mother's menstrual cycle and with the use of ultrasounds. Gestational age is calculated from the first day of the pregnant person's last menstrual period (LMP) to the present day. (Technically, gestational age includes the two weeks prior to conception, before the person is pregnant.)

Calendar calculations based on the last menstrual period assume the pregnant person has a regular 28-day cycle. Because that is often not the case, fetal ultrasounds help to provide more information to inform gestational age. In fact, first trimester ultrasounds of the embryo or fetus (up to and including 13 6/7 weeks of gestation) are the most accurate way to determine gestational age.

In the first trimester, ultrasound measures the length from the top of the head to the bottom of the baby's buttocks. This is the crown-rump length or CRL. In later stages of pregnancy, ultrasounds measure specific parts of the baby's body including the abdomen, head, and femur (thigh bone) to confirm gestational age and fetal growth.

Assisted Reproductive Technology and Gestational Age

In cases where assisted reproductive technology (ART) is used to conceive, the ART-derived gestational age is used to calculate the estimated date of delivery (EDD) or due date. For example, with in-vitro fertilization, doctors would use the age of the embryo and the date of the transfer along with ultrasounds to determine how far along the pregnancy is.

Gestational Age vs. Fetal Age

It is important to distinguish between gestational age and fetal age. While gestational age starts with the date of the LMP, embryonic age (and later fetal age) starts at the time of conception, when the sperm fertilizes the egg. Fetal age is two weeks behind gestational age and describes the actual age of the fetus.

Why Gestational Age Is Important

Gestational age is important because it helps guide prenatal care. It is an essential starting point providers use to:

  • Assess fetal growth
  • Determine the baby's due date
  • Schedule and evaluate prenatal test and screenings
  • Treat conditions that cause preterm and post-term birth

A full-term pregnancy is considered between 39 0/7 and 40 6/7 weeks. Babies born between 37 0/7 weeks through 38 6/7 weeks are considered premature and those born after 42 0/7 weeks are considered post-mature.

Gestational Age at Birth

At birth, a baby's condition is evaluated to determine how the baby relates to their gestational age. The doctor looks at their:

  • Hair condition
  • Head circumference
  • Height
  • Muscle tone
  • Posture
  • Reflexes
  • Skin condition
  • Vital signs
  • Weight

Based on those factors and how they compare to their calendar age, the baby is considered small for gestational age (SGA), large for gestational age (LGA), or appropriate for gestational age (AGA). In terms of weight, full-term infants who are AGA weigh on average between 5.5 pounds and 8.75 pounds. Babies who are small or large may be monitored more closely for complications.

Frequently Asked Questions

How do you determine gestational age?

Doctors use the date of the mother's last menstrual period and first trimester ultrasounds to determine gestational age. In pregnancies resulting from assisted reproductive technologies like IVF, the embryo's age and the date of conception are used to calculate gestational age.

How accurate are ultrasounds in determining gestational age?

Ultrasounds in early pregnancy are the most accurate way to determine gestational age, especially for pregnant people with a history of irregular menstrual cycles.

What is large for gestational age?

A baby is considered large for gestational age (LGA) if they weigh over 4,000 grams (8.75 pounds) at birth. They may also be taller or have a larger head than an average baby of the same age and sex.

What is small for gestational age?

A baby is considered small for gestational age (SGA) if they weigh less than 2,500 grams (5.5 pounds). They may also be shorter or less developed than a baby who is considered an appropriate weight for gestational age.

What is the difference between gestational age and weeks pregnant?

Gestational age is the time beginning with the pregnant person's last menstrual period (LMP) to the present (and as a result, includes the two weeks before conception occurred). It refers to how far along the pregnancy has progressed. Gestational age is the number most people use when describing how many weeks pregnant they are. Fetal age, on the other hand, starts at conception and only describes the age of the developing fetus.

A Word From Verywell

Gestational age is an important tool to help your doctor or midwife provide the best prenatal care throughout your pregnancy and to evaluate your baby's growth. It also allows you to follow along week by week as your baby develops. After all, what could be more exciting than imagining your baby sneezing or opening their eyes in the womb?

4 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. American College of Obstetricians and Gynecologists. Committee opinion no. 700: Methods for estimating the due date. Obstet Gynecol 2017;129:e150–4. doi:10.1097/AOG.0000000000002046

  2. American Pregnancy Association. Fetal development: First trimester.

  3. March of Dimes. What is full term?

  4. MedlinePlus. Gestational age.

By Krissi Danielsson
Krissi Danielsson, MD is a doctor of family medicine and an advocate for those who have experienced miscarriage.