Role of the Corpus Luteum During Pregnancy
The corpus luteum is essential for maintaining early pregnancy by producing progesterone in response to hCG stimulation, sustaining the endometrium until the placenta assumes steroidogenic function around 8-10 weeks of gestation. 1
Primary Function and Timeline
The corpus luteum serves as the critical source of progesterone during the luteal phase and the initial two-thirds of the first trimester of pregnancy. 2 This progesterone production is absolutely required for maintenance of normal pregnancy in mammals, as it provides an appropriate uterine environment for the developing conceptus. 3
Key Temporal Milestones
- hCG becomes detectable at levels >5 mIU/mL approximately 6-9 days after conception, representing the first hormonal signal from the placenta to the maternal system. 1
- The corpus luteum is rescued by hCG secreted from the blastocyst and trophoblast cells, which prevents its regression and maintains progesterone production. 1, 4
- Peak hCG levels occur at 8-12 weeks of gestation, followed by a steady decline through week 16 and beyond. 1
- Luteoplacental shift occurs around 8-10 weeks of gestation, when the placenta takes over steroidogenic function and the corpus luteum's role diminishes. 1, 2, 4
Mechanism of Action
The corpus luteum develops from residual follicular granulosal and thecal cells after ovulation. 3 During pregnancy, hCG from the developing pregnancy directly stimulates the corpus luteum to maintain progesterone secretion, preventing the luteolysis that would otherwise occur 12-14 days after ovulation. 2, 3
Progesterone production is closely related to the extensive vasculature surrounding and supplying the corpus luteum, with vascular supply being greatest at week 5 of pregnancy and declining continuously thereafter until week 11. 5 This vascular supply is essential to ensure delivery of LDL-cholesterol as substrate for steroidogenesis. 4
Clinical Significance Beyond Progesterone Production
The corpus luteum plays a broader role in maternal physiology during early pregnancy beyond simple progesterone production. 1 Specifically:
- The corpus luteum contributes to physiological changes in the maternal circulation during the luteal phase and early pregnancy. 1
- Impairment of corpus luteum function represents a potential causal or predisposing factor for preeclampsia, highlighting its importance in establishing normal pregnancy physiology. 1
Diagnostic Considerations
On transvaginal ultrasound, the corpus luteum typically appears as a <3-cm cystic lesion with a thick wall (with or without internal echoes), or as a rounded hypoechoic lesion. 6 A helpful diagnostic feature is the claw sign of partially surrounding ovarian parenchyma, which confirms an ovarian origin and helps distinguish the corpus luteum from other adnexal masses such as ectopic pregnancy. 6
The corpus luteum is located ipsilateral to an ectopic pregnancy in 70-80% of cases, making its identification important during evaluation for suspected ectopic pregnancy. 6, 7 However, the corpus luteum is typically hypoechoic compared to the hyperechoic rim of a tubal ring, which aids in differentiation. 6
Clinical Pitfalls
A common diagnostic challenge is differentiating an empty tubal ring of ectopic pregnancy from a corpus luteum, as both may demonstrate peripheral vascularity on color Doppler ultrasound. 6 Color flow is therefore not a discriminating feature between these entities. 6 The key distinguishing features are: