How Do You Evaluate Gestational Age? 4 Methods

Assessing gestational age is important because it gives information about:

• Developmental changes of the baby
• Health of the baby at birth
• Necessary testing and medical care for both mother and child (obstetrical care)
• Estimated date of delivery

1. Menstrual history

Traditionally, gestational age has been calculated using the date of the last menstrual period (LMP). This estimate is based on conception occurring on day 14 of the cycle. The error in this assumption is that the timing of ovulation varies substantially with respect to the menstrual cycle, both from cycle to cycle and from person to person. Gestational age tends to be overestimated when it is based on LMP. Menstrual dates have a 95% confidence interval of −27 to +9 days.

To make matters worse, 10%-45% of pregnant women are unable to offer reliable information about their LMP, and 18% of women with specific menstrual dates may have significant variations between menstrual and ultrasonographic timings.

Menstrual history accuracy should be questioned in women with a history of oligo-ovulation, such as those with polycystic ovarian syndrome. If conception occurred while using oral contraceptives or long-acting progestogens, LMP cannot be used because it has no link to ovulation time.

2. Clinical examination

Pelvic examination or abdominal palpation can be used to determine the size of the uterus. Size may be deceiving in cases of multiple pregnancy (twins/triplets), uterine fibroids, or a full bladder. Tape measurements of the symphysis-fundus height are useful up to 28-30 weeks of gestation, beyond which they become too inaccurate for dating.

The uterus remains a pelvic organ until about 12 weeks of pregnancy when it grows large enough to palpate on the abdomen right above the symphysis pubis. The uterine fundus is palpable at approximately 16 weeks, midway between the symphysis pubis and umbilicus; at approximately 20 weeks, it is felt at the umbilicus. The symphysis-fundus height in millimeters should coincide with the week of gestation after 20 weeks.

Conditions such as leiomyoma, obesity, multiple gestation, and other factors that affect the uterine size or capacity to palpate the uterus impair the diagnostic performance of physical examination-based gestational age estimation.

If there is no data available about the gestational age and if the expansion of the uterus is two fingerbreadths above the umbilicus, it confirms that the fetus is at a gestational age at the limit of viability. This is a rough estimation usually considered during any medical emergencies such as cardiac arrest of the mother.

3. Ultrasonography

Ultrasonography estimates gestational age depending on the following parameters:

• Size of the gestational sac
• Size of the embryo such as crown-rump length (CRL)
• Size of the fetal components such as the bones, abdomen, and skull

Ultrasounds are routinely performed on all pregnant women before 22 weeks of gestation. Early ultrasounds are considered accurate and the best way to determine gestational age over menstrual history or clinical examination. Moreover, it provides information about fetal development.

Ultrasound can be performed in two ways:

• Transabdominal scan (TAS)
• Transvaginal scan (TVS)

Sometimes, TAS may not be able to detect an intrauterine pregnancy, whereas TVS provides clear and precise images. As a result, TVS is advised for assessing the gestational sac and other early embryonic features such as the yolk sac and earliest identification of cardiac activity. TVS is more convenient than TAS for measuring CRL in the first trimester, although it is not more accurate for predicting gestational age.

TAS is used to measure anatomical segments of the fetus (biometry) in the second and third trimesters because the uterus expands into the mid and upper abdomen, and the fetus grows larger.

Although ultrasonography is considered superior than other methods of calculating gestational age, there are certain limitations:

• Improper images of the fetus due to the following:
  • Technical errors
  • Presence of fibroids in the uterus
  • Size and shape of the mother
• Multiple pregnancy (carrying more than one baby at a time)
• Fetal anomalies caused by the following:
  • Exposure to diseases or medicines
  • Genetic disorders
  • Delayed development due to malnutrition

4. Ballard score

The Ballard score assists clinicians in assessing gestational age. It is based on the newborn’s physical and neuromuscular maturity and can be used up to 4 days after birth. However, in practice, the Ballard score is usually used in the first 24 hours. 

Because physical features mature soon after birth, neuromuscular features become more constant with time. However, exposure to any disease and medicine can have an effect on neuromuscular features.

Because the Ballard score is only accurate within 2 weeks, it should only be used to assign gestational age when there is no reliable obstetrical information about the estimated date of confinement or when there is a significant discrepancy between the obstetrically defined gestational age and findings on physical examination.