What is the significance of a corpus luteum (ultrasound) on the ovary?

Corpus Luteum on Ovarian Ultrasound: Clinical Significance

The corpus luteum is a normal physiologic ovarian structure that appears as a <3 cm thick-walled cystic or hypoechoic lesion on ultrasound, representing the post-ovulatory follicle that produces progesterone to support early pregnancy. 1

Ultrasound Appearance and Identification

The corpus luteum demonstrates characteristic imaging features that aid in recognition:

• Typically appears as a <3 cm cystic lesion with a thick wall, with or without internal echoes in the central cystic component, or as a rounded hypoechoic lesion that may simulate a solid mass 1, 2

• The "claw sign" of partially surrounding ovarian parenchyma confirms ovarian origin and is a key diagnostic feature 3

• Location completely within the ovary is the most critical distinguishing characteristic, assessed by applying gentle pressure with the transvaginal transducer to determine if the mass and ovary move together 1

• The corpus luteum is hypoechoic compared to the hyperechoic rim of a tubal ring, which helps differentiate it from ectopic pregnancy 3

Physiologic Role in Early Pregnancy

Understanding the corpus luteum's function clarifies its clinical importance:

• Produces progesterone in response to hCG stimulation, maintaining the endometrium until the placenta assumes steroidogenic function around 8-10 weeks of gestation 3

• Becomes detectable on ultrasound immediately after ovulation, appearing cystic in approximately 50% of cases (mean 14.9 mm) and solid in the remainder (mean 12.8 mm) 4

• In the midluteal phase, cystic corpora lutea are larger (mean 18.2 mm) compared to solid forms (mean 13.4 mm) and correlate with higher estradiol and progesterone levels 4

Critical Diagnostic Distinction: Corpus Luteum vs. Ectopic Pregnancy

The most important clinical pitfall is misdiagnosing a corpus luteum as an ectopic pregnancy, as this can lead to inappropriate treatment with methotrexate or unnecessary surgery 1:

• Ectopic pregnancies are located ipsilateral to the corpus luteum in 70-80% of cases, making differentiation essential 1, 2

• Key distinguishing features include:

  • Intraovarian location (corpus luteum) versus extraovarian location (ectopic pregnancy) 1, 3
  • Hypoechoic appearance (corpus luteum) versus hyperechoic rim/tubal ring (ectopic pregnancy) 3
  • Presence of claw sign confirming ovarian origin (corpus luteum) 3

• To avoid misdiagnosing a corpus luteum as the rare ovarian ectopic pregnancy, a yolk sac or embryo should be present in the intraovarian thick-walled cystic structure 1

• Both structures may demonstrate peripheral vascularity on color Doppler, so location is more reliable than vascularity pattern for differentiation 3

Prognostic Significance in Early Pregnancy

The corpus luteum provides important prognostic information:

• Absence of a detectable corpus luteum on ultrasound is associated with nonviable pregnancy in 83% of cases, compared to 31% when a corpus luteum is present 5

• Decreasing corpus luteum volume between serial ultrasounds before 8 weeks' gestation is associated with 55% pregnancy loss rate, compared to 19% when volume increases 5

• The morphologic appearance (macrocystic, microcystic, or noncystic) has no predictive value for hormone concentration or pregnancy outcome 5

Association with Pelvic Pain

The corpus luteum can be a source of acute pelvic pain:

• Corpus luteum with adjacent ovarian stromal edema (measured as increased residual ovarian volume after subtracting corpus luteum volume) is significantly associated with pelvic pain 6

• High correlation exists between pain location and the side of the corpus luteum when pain duration is <14 days 6

• This presentation can mimic ovarian torsion on ultrasound (12.9% false-positive rate), particularly when associated with enlarged ovaries and peripherally displaced follicles 6

Clinical Management Implications

When a corpus luteum is identified on ultrasound:

• In pregnant patients with positive hCG and no intrauterine pregnancy, carefully evaluate whether any adnexal mass is truly extraovarian (concerning for ectopic) or intraovarian (likely corpus luteum) 1

• Luteal cysts >5 cm but <10 cm in premenopausal women warrant follow-up ultrasound in 8-12 weeks, as they typically resolve spontaneously 2

• Always use color Doppler in conjunction with grayscale imaging to fully characterize the corpus luteum and surrounding structures 7

• Fine-needle aspiration of ovarian masses is contraindicated due to risk of seeding if malignancy is present 2