Why does a 26-year-old primigravida at 40 weeks + 1 day have a fundal height 4 cm above expected and severe right hip pain—could the large (84th percentile) fetus and a 2.5 × 5 cm benign‑appearing right iliac lymph node explain these findings, or are other causes more likely?

Large-for-Gestational-Age Fetus with Right Hip Pain at Term

The fundal height measuring 4 cm ahead at 40+1 weeks is most likely explained by the large fetus (84th percentile) combined with normal amniotic fluid and a primigravida body habitus, while the severe right hip pain is probably musculoskeletal strain from the large gravid uterus rather than the 2.5×5 cm lymph node. 1

Why the Belly Measures Large

The primary explanation is the appropriately grown large fetus at the 84th percentile. 1 At term, fundal height in centimeters typically corresponds to gestational age in weeks between 16-36 weeks, but this relationship becomes less reliable after 36 weeks due to fetal engagement and descent. 2

Normal Physiologic Variation at Term

• "Lightening" (fetal head descent into the pelvis) typically occurs in primigravidas 2-4 weeks before delivery and can actually cause fundal height to decrease from 36 cm down to approximately 32 cm. 2 The fact that this patient's fundal height is increased rather than decreased suggests the baby has not yet engaged ("baby is floating"), which is keeping the fundus higher in the abdomen. 2

• Primigravidas often appear "all belly" because nulliparous abdominal wall musculature is tighter and directs the gravid uterus more anteriorly rather than allowing lateral expansion. 2

Factors Contributing to Increased Fundal Height

• A fetus at the 84th percentile for growth is appropriately large and will naturally produce a larger fundal height measurement. 1 Serial ultrasound showing accelerated abdominal circumference growth (>1.2 cm/week in the third trimester) is the hallmark of large-for-gestational-age fetuses. 3

• Normal amniotic fluid volume contributes to appropriate uterine size; polyhydramnios would be a concern if AFI were elevated, but your ultrasound showed normal fluid. 1

• The non-engaged fetal head ("baby is floating") prevents the normal late-pregnancy fundal height drop and maintains a higher measurement. 2

What Could Be Causing the Right Hip Pain

The severe right-sided hip pain is most likely musculoskeletal strain from the large gravid uterus and weight of the fetus, not the lymph node. 1

Why the Lymph Node Is Unlikely the Culprit

• A 2.5×5 cm "likely benign" lymph node in the right iliac region is too small and too superficial to cause severe hip pain through mass effect alone. The node would need to be significantly larger or directly compressing the obturator nerve, femoral nerve, or lumbosacral plexus to produce severe symptoms—none of which were reported on the ultrasound. 1

• Benign reactive lymphadenopathy is common in pregnancy due to physiologic immune changes and increased lower extremity lymphatic drainage. 1

More Likely Musculoskeletal Causes

• Sacroiliac joint dysfunction and pelvic girdle pain are extremely common in late pregnancy, especially with a large fetus and primigravida status. The weight of an 84th-percentile fetus creates significant anterior pelvic tilt and sacroiliac joint strain. 1

• Round ligament pain, though typically bilateral, can be asymmetric and severe with a large uterus. 1

• Pressure on the lumbosacral plexus from the fetal head (even if not engaged) can cause referred hip and lateral thigh pain. 1

What to Do on Friday's Ultrasound

Essential Assessments

• Confirm estimated fetal weight remains on an appropriate growth trajectory (not accelerating into macrosomia >90th percentile) and document abdominal circumference growth velocity. 1, 3 If AC growth exceeds 1.2 cm/week, consider screening for gestational diabetes if not already done. 3

• Reassess amniotic fluid volume to ensure it remains normal; polyhydramnios would suggest fetal hyperglycemia or other pathology. 1

• Document fetal presentation and station—if the head remains unengaged at 40+1 weeks, consider whether cephalopelvic disproportion might be present. 2

• Re-evaluate the lymph node: measure it again and assess for interval growth, which would be concerning for malignancy (though unlikely given "likely benign" characterization). 1

Clinical Decision Points

• If the fetus remains appropriately grown at 84th percentile with normal fluid and reassuring testing, proceed with expectant management or induction per your routine post-dates protocol. 1 There is no indication for early delivery based on size alone unless EFW exceeds 4500 g (macrosomia threshold). 3

• If the lymph node has grown significantly or demonstrates concerning features (irregular borders, necrosis, vascular invasion), refer to general surgery or oncology for further evaluation—but this is unlikely given the initial benign appearance. 1

Common Pitfalls to Avoid

• Do not attribute the large fundal height to pathology (polyhydramnios, twins, molar pregnancy) when ultrasound has already confirmed a singleton pregnancy with normal fluid and appropriate fetal growth. 1 The measurements are consistent with a large but healthy fetus in a primigravida.

• Do not over-investigate the lymph node unless it demonstrates interval growth or concerning features on repeat imaging. 1 Benign reactive adenopathy is common and does not require intervention.

• Do not dismiss the hip pain as "just pregnancy"—provide appropriate analgesia (acetaminophen, physical therapy referral for pelvic stabilization exercises) and reassurance that it will resolve postpartum. 1

• Recognize that fundal height measurements become unreliable after 36 weeks due to fetal engagement, maternal body habitus, and measurement variability. 2 Ultrasound biometry is the gold standard for assessing fetal size at term. 1