Fundal height is a simple measurement utilized during prenatal care to estimate the growth of the fetus inside the womb. It is defined as the distance, in centimeters, from the top of the pubic bone to the highest point of the uterus, known as the fundus. This quick, non-invasive method allows healthcare providers to track the progression of a pregnancy during routine office visits.
Why Fundal Height Is Measured
The primary purpose of measuring fundal height is to consistently monitor fetal growth over time. Tracking this measurement helps providers screen for potential deviations in growth, indicating if the fetus may be growing slower or faster than expected. The measurement also offers clues about the volume of amniotic fluid present in the uterus.
Measurement typically begins during the second trimester, around 20 weeks of gestation, when the uterus has risen high enough above the pelvis to be reliably measured externally. By charting the fundal height at each subsequent prenatal appointment, providers establish a pattern of growth. If the measurement falls outside the expected range, it can indicate the need for further, more detailed evaluation, such as an ultrasound.
Step-by-Step Measurement Technique
The process of measuring fundal height begins with the patient lying flat on their back on the examination table. Before the measurement is taken, the patient may be asked to empty their bladder, as a full bladder can potentially alter the reading by a few centimeters. The healthcare provider first locates the two anatomical landmarks: the superior border of the symphysis pubis and the uterine fundus.
The symphysis pubis is the joint where the pubic bones meet. The fundus is identified by palpating the abdomen to find the firm, rounded top edge of the uterus. A flexible, non-stretchable measuring tape, calibrated in centimeters, is used for the reading. The zero end of the tape is placed precisely at the top edge of the symphysis pubis.
The tape is then stretched lengthwise over the curve of the abdomen to the location of the fundus. It is important to ensure the tape measure follows the contour of the abdomen without pressing down too hard. The measurement is read and recorded in centimeters, representing the distance between the two landmarks. This numerical result is then plotted on a growth chart to compare it against previous measurements and expected norms.
Understanding the Measurements
The general expectation for fundal height is that the measurement in centimeters will roughly correspond to the number of weeks of gestation. This correlation is most consistent and reliable between 20 and 36 weeks of pregnancy. For instance, a person who is 28 weeks pregnant would typically have a fundal height measurement of approximately 28 centimeters.
It is widely accepted that the measurement can naturally vary by plus or minus two to three centimeters from the gestational week. Therefore, a reading of 26 to 30 centimeters at 28 weeks is generally considered within the normal range. The measurement is used as an estimate of growth, not as a precise indicator of fetal size or gestational age. Factors like the position of the fetus or the mother’s body shape contribute to this normal margin of error.
Causes of Unexpected Results
When the fundal height measurement is consistently more than three centimeters larger or smaller than the expected gestational age, it is considered a deviation that warrants further attention. This unexpected result does not automatically signify a problem, as several non-concerning factors can influence the reading. The provider will use this finding to decide whether an ultrasound or other tests are necessary to investigate the cause.
Measuring larger than expected can be attributed to a twin or multiple pregnancy, which increases the overall uterine size. A larger measurement may also be caused by polyhydramnios, a condition characterized by an excessive amount of amniotic fluid.
Other reasons include a fetus lying in a breech position or a larger-than-average fetus (macrosomia). The measurement can also be influenced if the patient has a higher body mass index or looser abdominal muscles.
Conversely, measuring smaller than expected may be due to the fetus descending into the pelvis, which typically happens in the weeks leading up to delivery. A smaller measurement may also occur if the patient has a smaller stature or very toned abdominal muscles. More serious reasons for a reduced measurement include oligohydramnios (low volume of amniotic fluid) or intrauterine growth restriction, where the fetus is not growing at the expected rate.