4 ways to evaluate gestational age
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)
- Transvagin*l 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.
FAQs
The 3 basic methods used to help estimate gestational age (GA) are menstrual history, clinical examination, and ultrasonography.
What are the methods to assess gestational age? ›
The 3 basic methods used to help estimate gestational age (GA) are menstrual history, clinical examination, and ultrasonography.
What are the 4 measurements that must be done to estimate fetal age? ›
In the second and third trimesters, fetal head, body, and extremity measurements have been commonly used to assess gestational age. Those parameters most commonly measured include biparietal diameter,20,21,22,23,24,25,26,27,28 head circumference,29,30 abdominal circumference,31,32,33,34,35 and femur length.
What is the most accurate method to determine gestational age? ›
Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age 3 4 7 8 9 10.
How do you assess fetal gestational age? ›
A combination of history, physical exam, early sonography in the first trimester, and prenatal assessments are all essential to obtaining a more accurate gestational age. Different techniques may be more useful depending on how far along the pregnancy has progressed.
How do you test for gestational age? ›
When periods are regular, the menstrual history is a relatively reliable method of determining estimated date of delivery. When other information is lacking, ultrasonographic measurements of the fetus in the first trimester give the most accurate estimate of gestational age.
What is the best method for assessing a newborn's gestational age? ›
The principal method used to postnatally estimate gestational age (GA) is the new Ballard score, which combines physical and neurologic criteria (figure 1). The new Ballard score is easier to administer than the longer Dubowitz method.
How to calculate gestation age? ›
To calculate your estimated due date, which should be around 40 weeks after your LMD, add 7 days to your LMD, and then count 3 months back and add one year. For example, if the LMD was March 11, 2021, add 7 days, (March 18, 2021), then subtract 3 months (December 18, 2020) and add 1 year (December 18, 2021).
What is a common method of estimating gestational age of a newborn is to assess? ›
The Ballard score is commonly used to determine gestational age. Here's how it works: Scores are given for 6 physical and 6 nerve and muscle development (neuromuscular) signs of maturity. The scores for each may range from -1 to 5.
What is the rule for determining gestational age? ›
In real life, determining gestational age can be challenging. The estimated delivery date is calculated by adding 280 days to the first day of the last menstrual period. An alternative method of determining the due date is to add 7 days to the LMP, subtract three months, and add one year.
Gestational age vs fetal age
While gestational age is measured from the first day of your last menstrual period, fetal age is calculated from the date of conception. This is during ovulation, which means that fetal age is about two weeks behind gestational age. This is the actual age of the fetus.
Which measurement is the most reliable indicator of gestational age in the first trimester? ›
If a fetal pole (embryo less than ten weeks) or fetus (greater than ten weeks) is visible, further gestational age estimations are obtained by either crown-rump length or biparietal diameter. In the first trimester, crown-rump length is the most accurate metric to determine gestational age.
What is the most accurate due date method? ›
Typically, an ultrasound is used to confirm your due date (based on your estimated due date calculated from your last menstrual period), and “once a due date has been selected, it does not change no matter how many additional ultrasound exams you may have during your pregnancy,” states ACOG.
What are the clinical methods of estimating gestational age? ›
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)
What is the rapid assessment of gestational age? ›
A simple scoring system based on skin colour, skin texture, breast development, and ear firmness that can be performed even in an ill baby without manipulation or movement enables gestational age to be estimated to within +/- 15 days (95% confidence limits) at any time in the first 2 days of life.
How do you document gestational age? ›
Gestational age should be rounded off to the nearest completed week, not the following week. For example, an infant born on the 5th day of the 36th week (35 weeks and 5/7 days) is at a gestational age of 35 weeks, not 36 weeks. Gestational age should be documented by the clinician as a numeric value between 1-50.
Which of the following can be used to assess gestational age? ›
An examination called the Dubowitz/Ballard Examination for Gestational Age is often used. A baby's gestational age often can be closely estimated using this examination. The Dubowitz/Ballard Examination evaluates a baby's appearance, skin texture, motor function, and reflexes.
Which method of assessment to determine the age of gestation in months? ›
Ultrasonography may be used to estimate gestational age. For determining gestational age in the first trimester, the crown-rump length is most accurate.
What is the easiest way to calculate gestational age? ›
To calculate your gestational age, or many weeks pregnant you are, you need to know the date of your Last Menstrual Period (LMP). Starting from this date, count the weeks up to today's date. This number will correspond to the number of weeks pregnant.
