Gestational Age

What is gestational age?

Gestational age is the length of time that a fetus grows and develops inside the mother’s uterus. It is the common term to describe how far along the pregnancy is.

Gestational age is crucial to get accurate information about your pregnancy, in which trimester you are and also to know your expected due date of delivery. The more accurate your gestational age, the more accurate your due date will be.

Gestational age is divided into two periods

1.       Embryonic period: Week 5 (this week embryo implants in your uterus) to week 10 of pregnancy

2.       Fetal period: Week 10 to birth. During fetal period, important developments like all vital organ formation happens.

How to calculate gestational age?

Gestational age is measured in weeks, from the first day of the woman’s last menstrual cycle to the current date. A normal pregnancy can range from 38 to 42 weeks. Gestational age of 40 weeks can be considered as a full time pregnancy. A child born before 37 weeks of gestation is considered to be a premature baby.

The estimated due date             

The human egg is capable of fertilization for only 12 to 24 hours after ovulation, so the date of ovulation may be taken as being the date of conception.

A woman is most likely to become pregnant if she had sex on the day of ovulation. Sometimes conception can also take place if live sperm is still in her reproductive tract on the day of ovulation (even though she had sex up to 5 days before ovulation).

The estimated due date (EDD) is the date that spontaneous onset of labor is expected to occur. The due date may be estimated by adding 280 days (9 months and 7 days) to the first day of the last menstrual period (LMP). The accuracy of this method depends on accurate recall by the mother, assumes regular 28 day cycles, and that ovulation and conception occurs on day 14 of the cycle. Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks.

If the date of conception is known exactly, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception.

Methods to calculate gestational age

Ultrasound:

Ultrasound is considered to be the gold standard and the most accurate method for calculating gestational age of the fetus. Ultrasound uses the size of the fetus to determine the gestational age. While looking at a live picture, doctors can determine a baby's sex, check on their health, measure their development, and usually determine their fetal age.

Other methods for estimating the gestational age include pelvic examination supported by proper menstrual records, doppler ultrasonography (fetal heart rate can be heard by ultrasonography through which gestational age can be correlated). However these methods are less accurate.

Gestational age measurement after birth

Gestational age is measured after birth by looking at the baby's weight, length, head circumference, vital signs, reflexes, muscle tone, posture, and the condition of the skin and hair.

If the baby's gestational age findings after birth match the calendar age, the baby is said to be appropriate for gestational age (AGA). AGA babies have lower rates of problems and death than babies that are small or large for their gestational age. The weight for full-term infants that are born AGA will most often be between 2,500 grams and 4,000 grams.

Infants weighing less are considered Small for Gestational Age (SGA). Infants weighing more are considered Large for Gestational Age (LGA)

Gestational age helps guiding prenatal care. In addition, it yields an expected due date and is the method most doctors use for dating a pregnancy. 

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