The term “fundus” is a Latin anatomical term referring to the base or bottom of a hollow organ, typically the part farthest from the opening. Although applied to several organs, the fundus of the uterus holds particular medical significance. During pregnancy, tracking the position and growth of the uterine fundus is a standard, non-invasive method used by healthcare providers. Monitoring its location provides important diagnostic information about the progression of the pregnancy and the health of the developing fetus.
Defining the Fundus and Its Location
The fundus is anatomically defined as the superior, rounded portion of a hollow organ, situated opposite the organ’s outlet. In the uterus, the fundus is the broad, uppermost part located above the openings where the fallopian tubes connect. This region represents the roof of the uterine cavity.
In a non-pregnant individual, the uterus is a relatively small, pear-shaped organ that remains deep within the pelvic cavity. It is positioned between the urinary bladder and the rectum. As pregnancy advances, the uterus expands significantly, causing the fundus to rise out of the pelvis and become an abdominal organ.
How Fundal Height is Measured
The technique used to monitor uterine growth during pregnancy is called fundal height measurement, sometimes referred to as McDonald’s rule. This measurement is typically performed at every prenatal visit starting around the 20th week of gestation. The procedure begins with the patient lying on her back and asked to empty her bladder, as a full bladder can alter the reading.
The clinician first palpates the abdomen to locate two specific landmarks. The starting point is the symphysis pubis, the firm joint at the top of the pubic bone. The ending point is the fundus, the highest palpable point of the uterine wall.
A non-elastic measuring tape is extended from the top edge of the symphysis pubis, over the curve of the abdomen, to the top of the fundus. The resulting distance is recorded in centimeters. This simple measurement serves as a screening tool to estimate fetal growth and verify gestational age.
Tracking Pregnancy Milestones by Fundal Position
The location of the uterine fundus changes predictably throughout pregnancy, offering anatomical milestones that correlate with gestational age. Around 12 weeks of gestation, the fundus is felt just above the symphysis pubis, meaning the uterus is no longer entirely confined to the bony pelvis.
As pregnancy progresses, the fundus continues its ascent into the abdominal cavity. By approximately 20 weeks, the fundus is typically located at the level of the umbilicus. This landmark is significant because, until about 36 weeks, the measurement in centimeters should roughly match the number of weeks of pregnancy (plus or minus two or three centimeters).
This correlation provides a quick assessment of whether the fetus is growing at an expected rate. For instance, a person 28 weeks pregnant would expect a measurement of 25 to 31 centimeters. If the fundal height is consistently measured as significantly smaller or larger than expected, further investigation is needed.
A measurement notably smaller than gestational age may signal conditions such as intrauterine growth restriction (IUGR) or oligohydramnios (low amniotic fluid volume). Conversely, a larger-than-expected measurement can suggest polyhydramnios (excess amniotic fluid) or the presence of multiple fetuses.
The fundus reaches its highest point in the abdomen around 36 weeks of gestation, often touching the xiphoid process. After this time, the measurement may slightly decrease as the fetal head descends into the pelvis in preparation for delivery, a process known as lightening. Therefore, fundal height measurements become less accurate for estimating gestational age in the final weeks of the third trimester.
The Fundus in Non-Uterine Anatomy
The anatomical concept of the fundus is applied to several other hollow organs in the body. The stomach, for instance, has a fundus, which is the dome-shaped upper part that bulges above the esophageal opening. Similarly, the fundus of the gallbladder is the rounded, closed end that projects downward from the liver’s underside.
The urinary bladder also features a fundus, referring to the posterior, inferior surface that faces the rectum.
Postpartum Involution
After childbirth, the uterine fundus undergoes involution, contracting rapidly to return to its pre-pregnancy size. Immediately after delivery, the fundus is typically palpable near the level of the umbilicus. It then descends into the pelvis at a rate of approximately one centimeter each day. By about 10 to 14 days postpartum, the fundus is no longer palpable above the symphysis pubis.