What Does Gestational Age Mean in Pregnancy?

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

Gestational age helps 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.

Learn more about gestational age, why it matters, and how it's calculated.

Determining Gestational Age

Parents don't always know exactly when conception occurred. Health care providers can determine weeks of gestation by looking at the pregnant person's menstrual cycle and by using ultrasounds.

Using the menstrual cycle: 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.)

Using ultrasounds: Calendar calculations based on the last menstrual period assume the pregnant person has a regular 28-day cycle. Because that's often not the case, fetal ultrasounds provide more information to gauge gestational age. 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, especially for pregnant people with a history of irregular menstrual cycles.

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, health care provider's would use the age of the embryo and the date of the transfer, along with ultrasounds, to determine how far along the pregnancy is rather than the LMP.

Gestational Age vs. Fetal Age

It's 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 Matters

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

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

Gestational Age at Birth

At birth, a baby's condition is evaluated using a variety of screening tests to determine their overall health, and their health relates to their gestational age. Your health care provider looks at the newborn's:

• Hair condition
• Head circumference
• Hearing
• Height
• Hemoglobin
• Hormones
• Metabolism
• 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.

What is small for gestational age (SGA)?

A baby is considered 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 large for gestational age (LGA)?

A baby is considered 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.