Lightening in Late Pregnancy: Fundal Height Changes

Understanding Lightening

• Lightening refers to the descent of the fetal head into the maternal pelvis during late pregnancy, which causes the uterine fundus to drop in height, as noted by the American Heart Association 1

Normal Fundal Height Progression

• The fundus reaches the level of the umbilicus at approximately 20 weeks gestation, according to the American College of Cardiology 2, 1
• In the last month of pregnancy (after 36 weeks), there may be a diminution of fundal height from 36 cm down to approximately 32 cm as the fetal head engages into the pelvis, as reported by the American Heart Association 1

The Mechanism of Lightening

• This descent occurs when the fetal presenting part (usually the head) descends into the maternal pelvis in preparation for labor, as described by the American College of Cardiology 1
• The engagement of the fetal head causes the fundus—the top portion of the uterus—to drop lower in the maternal abdomen, resulting in a noticeable change, particularly in primigravidas (first-time mothers), typically occurring in the last 2-4 weeks before delivery, as noted by the American Heart Association 1

Clinical Significance

• The drop in fundal height during lightening is a normal physiological process and should not be confused with pathological causes of decreased fundal height, as emphasized by the American College of Cardiology 1

Important Distinctions

• The cervix does not "drop" in height—it undergoes effacement (thinning) and dilation during labor, as distinguished by the American Heart Association 1
• Only the fundus (the uppermost portion of the uterus) demonstrates measurable descent during lightening, as clarified by the American College of Cardiology 1

Assessment Considerations

• Fundal height measurements can be affected by maternal body habitus, abdominal distention, and increased body mass index, as considered by the American Heart Association 1
• The symphysis-fundal height measurement in centimeters approximately corresponds to gestational age in weeks between 16-36 weeks gestation, as measured by the American College of Cardiology 1

Uterine Fundal Height Measurement

Clinical Landmarks for Gestational Age Estimation

• The American Heart Association guidelines provide that the uterus becomes palpable above the pubic symphysis at 12 weeks gestation, and reaches the level of the umbilicus at 20 weeks, and the xiphisternum at 36 weeks 3
• The uterus size and position can be used to estimate gestational age, with the uterus being approximately the size of a grapefruit and beginning to protrude from the pelvis at 10-12 weeks gestation 3

Factors Affecting Fundal Height Assessment

• Body habitus, such as abdominal distention and increased body mass index, can significantly reduce the reliability of fundal height as a predictor of gestational age 3
• Multiple gestations and fetal position, such as a non-longitudinal lie, can also make fundal height measurements less reliable 3

Reliability of Fundal Height Measurement

• The symphysis-fundal height measurement becomes more reliable between 16-36 weeks gestation, when the height in centimeters approximately corresponds to gestational age in weeks 3, 4

Symphysis-Fundal Height Measurement Guidelines

Introduction to SFH Measurements

• The American College of Obstetricians and Gynecologists recommends initiating symphysis-fundal height (SFH) measurements at 24 weeks gestation and continuing serially throughout the third trimester at every antenatal visit 5, 6, 7, 8

Guideline Consensus on Timing

• The United Kingdom guidelines recommend serial fundal height measurements on customized charts starting at 24 weeks gestation 5, 6, 7, 8
• The New Zealand guidelines recommend beginning SFH measurements at 26 weeks gestation 5, 6, 7, 8
• The Canadian, Irish, United States, and French guidelines recommend serial fundal height measurements from approximately 22-24 weeks onward 5, 6, 7, 8

Rationale for This Timing

• The measurement becomes clinically reliable between 16-36 weeks gestation, when the height in centimeters approximately corresponds to gestational age in weeks 5, 6

Measurement Technique

• All guidelines recommend using a tape measure to assess SFH, with 50% of guidelines specifically recommending plotting measurements on customized growth charts rather than population-based charts 5, 6, 7, 8
• The measurement should be performed at every antenatal visit once initiated 5, 6, 7, 8

Clinical Triggers for Ultrasound Evaluation

• Guidelines recommend ultrasound assessment when fundal height is <10th centile on customized charts 5, 6, 7, 8
• Guidelines recommend ultrasound assessment when fundal height shows slow or static growth (change in measurement <5 mm over 14 days) 7, 8
• Guidelines recommend ultrasound assessment when fundal height is >3 cm less than gestational age in weeks (using McDonald rule) 5, 6

Important Caveats

• Fundal height measurements are unreliable in women with obesity 5, 6
• Fundal height measurements are unreliable in women with fibroid uterus 5, 6
• Fundal height measurements are unreliable in women with abdominal distention or increased body mass index 9

Special Consideration: Growth Assessment Protocol

• The Growth Assessment Protocol (GAP) in the United Kingdom, which incorporates standardized SFH measurement training and customized growth charts starting at 24 weeks, has been associated with increased detection of small-for-gestational-age babies and reduction in stillbirth 5, 6