Fundal height is a simple measurement routinely taken during the second half of pregnancy to monitor fetal development. The measurement is the distance, recorded in centimeters, from the top edge of the pubic bone to the very top of the uterus, known as the fundus. This non-invasive screening method provides a quick estimate of fetal growth and how the size of the uterus corresponds to gestational age.
The Clinical Purpose of Fundal Height Tracking
Healthcare professionals use fundal height tracking as an initial, inexpensive method to screen for potential growth deviations. By assessing this measurement over time, providers determine if the fetus is growing consistently along the expected curve. A measurement outside the typical range suggests that further investigation is necessary.
Since it is a screening tool, a single reading outside the normal range does not diagnose a problem. Instead, it indicates the need for more definitive testing, such as an ultrasound. Fundal height monitoring is a standard component of prenatal care, beginning around the 20th week.
Step-by-Step Measurement Procedure
The measurement process begins with the patient lying flat or slightly tilted, and ensuring their bladder is empty. A full bladder can alter the measurement by several centimeters. The healthcare provider first locates two specific anatomical landmarks on the abdomen.
The first landmark is the symphysis pubis, the joint at the top of the pubic bone, which serves as the starting point. The provider then gently palpates the abdomen to locate the fundus, the uppermost portion of the uterus. This palpation determines the highest point of the uterus.
A flexible, non-stretchable tape measure, marked in centimeters, is used for the actual measurement. The zero end of the tape is placed firmly at the upper border of the symphysis pubis. The tape is then extended up and over the curve of the abdomen, following the longitudinal axis of the uterus, until it reaches the fundus. The resulting number is recorded in centimeters, representing the fundal height for that prenatal visit.
Interpreting the Measurement Results
The general rule for interpreting fundal height is that after the 20th week of pregnancy, the measurement in centimeters should approximately equal the number of weeks of gestation. For instance, a person who is 28 weeks pregnant would typically have a fundal height of about 28 centimeters. A measurement is generally considered within the expected range if it falls within two or three centimeters of the gestational age.
A measurement that is consistently three centimeters or more smaller than the expected week of gestation may suggest fetal growth restriction. This deviation could also indicate oligohydramnios, which is a reduced volume of amniotic fluid. Conversely, a measurement that is three centimeters or more larger than expected might suggest the presence of multiple fetuses, such as twins, or polyhydramnios, meaning too much amniotic fluid.
These significant deviations prompt the need for follow-up diagnostic testing, most often an ultrasound, to gain a more precise assessment of fetal size and amniotic fluid volume. An ultrasound provides a more accurate picture of how the fetus is growing and can help confirm or rule out clinical concerns. A deviation may also suggest that the estimated due date is incorrect, which the ultrasound can help clarify.
Factors That Can Influence Accuracy
Several non-pathological factors can cause the fundal height measurement to deviate from the expected range, confirming it is not a definitive diagnostic tool.
Factors Affecting Measurement
- Maternal body habitus: A higher body mass index (BMI) or significant abdominal muscle tone can make accurate palpation difficult, leading to a less reliable reading.
- Fetal position: If the fetus is positioned transversely or has descended (“engaged”) into the pelvis close to term, the measurement may be shorter than expected.
- Uterine characteristics: Conditions such as fibroids or a uterus stretched from prior pregnancies can cause the measurement to be inaccurately large.
- Provider variation: Differences in technique or the pressure applied during palpation between healthcare providers can lead to inconsistencies in the recorded number.
These factors underscore that fundal height is a screening measure used to identify pregnancies that may benefit from additional surveillance.