Luteinized Unruptured Follicle (LUF) Syndrome

Luteinized Unruptured Follicle (LUF) syndrome occurs when the dominant follicle begins producing progesterone but never ruptures to release its egg. Hormone tests suggest that ovulation took place, and periods usually come on a normal schedule. Yet no egg can be fertilized, because no egg was released.

LUF syndrome is an overlooked contributor to infertility. Only ultrasound tracking of the follicle can detect it. Routine care rarely includes this tracking. One study examined RRM care at two family medicine clinics in New England. It found that this type of tracking was seldom used, which helps explain why LUF syndrome so often goes unnoticed.¹

The diagnosis requires ultrasound. Repeat scans follow the dominant follicle as it grows to its expected mature size. The scans then show signs of luteinization, the start of progesterone production. The follicle appears brighter on the scan (increased echogenicity), and its wall changes.

However, the follicle does not collapse and release its fluid, as it would in a true rupture. It stays in place. After a normal ovulation, an empty, collapsed sac would be present instead.

LUF syndrome may occur in some cycles and not in others. This makes the diagnosis harder. One monitored cycle with a normal ovulation does not rule it out.

LUF cycles still produce progesterone. For this reason, charting the luteal phase alone cannot tell them apart from cycles in which the egg is released. Only ultrasound can tell them apart. Finding LUF syndrome turns "unexplained" infertility into a condition that can be diagnosed and addressed.