What is fundus measurement in pregnancy?

Fundus measurement was first introduced in the late 1970s and became popular in the late 1980s. Given its simplicity and the very low cost of the procedure, it has been widely used by many health professionals, especially midwives, in the second part of pregnancy.

If you are pregnant, your gynecologist or midwife will probably have measured your fundus. It is a measurement that is generally performed starting from the 20th week of pregnancy, and although it is a simple method, it can give important information.

The measurement of the uterine fundus is performed using a normal tape measure , and allows for an initial assessment of the size, growth rate and position of the baby.

The measurement should be characteristic of the gestational period in which you are. In fact, each week corresponds to a certain interval of normality measurements. For example, at 20 weeks pregnant, your fundus measurement should be between 18 and 22 centimeters.

As your belly grows, your bottom height should also increase, but there are a few reasons why your measurement may be higher or lower than expected. The baby may be larger or smaller than average, the baby may be in a breech or lateral position, or the woman may have gestational diabetes.

If your midwife or doctor notices an abnormality in the measurement, you will need to undergo an ultrasound scan to understand the cause. Although ultrasounds do not give 100% accurate information, they are certainly more precise than measuring the fundus.

The measurement of the uterine fundus remains however  very important in all those areas of the world where ultrasound machines are not available .

The information that is obtained from the measurement of the uterine fundus

Fundus measurement was first introduced in the late 1970s and became popular in the late 1980s. Given its simplicity and the very low cost of the procedure, it has been widely used by many health professionals, especially midwives, in the second part of pregnancy.

The main objectives of measuring fundal height during pregnancy are to detect:

• Unusually young children
• Unusually large children
• Multiple pregnancies (relevant before the diffusion of ultrasound machines)
• gestational age

All these indications are useful in those situations where there is no availability of ultrasounds or frequent ultrasounds.

As for the gestational age, it was seen in a study published in 2018 that the more precise the dating, the more numerous the measurements are, especially after 20 weeks of pregnancy.

The error on the dating obtained with the measurement of the uterine fundus is about 2 weeks.

A 2016 observational study created an international standard of symphysis-fundus measurement using measurements obtained in healthy pregnancies from 8 countries.

The figure shows the growth curve of the measurement of the uterine fundus with respect to the gestational age (starting from 14 weeks of pregnancy), at the 3rd, 10th, 50th, 90th and 97th percentiles.

For a quick reference, think that the measurement of the fundus should more or less correspond to the weeks of pregnancy in which you are .

For example, at 20 weeks of pregnancy you should measure 20 cm, at 25 weeks 25 cm, and so on (up to about 36-37 weeks). However, fundal height measurements may vary 2 to 4 cm more or less than expected, for various reasons, even in normal pregnancies.

As far as fetal measurements are concerned, fundus measurement simply gives a very superficial indication as it has an accuracy of less than 50%.

With ultrasounds, the weight is wrong by about 10-15% (on average 400-600 grams).

How to measure the fundus

The expectant mother should be placed on her stomach or in a semi-recumbent position (45 degree angle) on a firm surface, with an empty bladder.
Bottom height measurements should be taken using a normal tape measure.
After palpation with the hands to locate the fundus, measurement should start from the variable point: the highest point of the uterus which is the fundus.
By fixing the end of the measuring tape to the bottom, the measuring tape is stretched along the abdomen, passing through the navel to stop at the top of the symphysis pubis which is the most easily identifiable reference point.

You take the measurement and plot it on the graph to monitor growth over time.

For women with a body mass index > 35,  polyhydramnios , or with multiple or large fibroids, fundus measurement is not accurate and should not be done.

Differences in measurement

The reasons for having a lower or higher than expected background measurement can be medical (such as gestational diabetes or failure to thrive) or due to secondary causes such as:

• Child position. Measurements of fundal height are standardized to measure babies in the cephalic (head down) position. A baby in the breech position may give more degree measurements, while a baby in the transverse (lying on its side) or oblique (lying diagonally) position will give lower measurements
• Physical differences in height and weight of each woman . Bottom height can vary between very tall women and very short women. Even women who are very thin may have smaller measurements, while women who are overweight may have larger measurements.
• Differences between first and subsequent pregnancies . Women who are having their first baby tend to have tighter abdominal muscles and less loose ligaments; these factors keep the uterus in the expected position, giving the expected measures. Women who are expecting their second or subsequent baby tend to carry their belly lower and may be smaller in size than their first pregnancy.
• Familiarity differences. Some women tend to have smaller babies and some women tend to have older babies. A baby’s size will tend to follow the woman’s family history and not that of her partner (contrary to popular belief).
• Operator-employee differences. We have seen that the accuracy of bottom height measurements is affected by the operator taking the measurement . Each healthcare professional may measure slightly differently, affecting your overall results.

All of the factors listed above must be taken into consideration when interpreting the fundus measurement. The measurement may be okay if it is consistently lower or higher at each visit (e.g. 22cm at 24 weeks and then 26cm at 28 weeks or 30cm at 28 weeks and 34cm at 32 weeks).

The concern arises when there is no homogeneity in the growth trend. For example when 24 cm was measured at 24 weeks and only 25 cm at 28 weeks: in this case the baby has not shown progressive growth over the four weeks. Consequently, an ultrasound is needed to see how it is growing and if there are any problems.