Corpus luteum cyst in early pregnancy: what it is and what to expect

The NHS describes functional ovarian cysts as cysts that develop as part of the menstrual cycle; they are usually harmless, short-lived and the most common type. A Scottish guideline from NHS Borders describes the corpus luteum cyst as one of the common causes of an adnexal mass (a lump beside the womb) in pregnancy. The corpus luteum is the structure that forms on the ovary after an egg is released, and a corpus luteum cyst is one that has filled with fluid.

Pathological cysts, caused by abnormal cell growth, are much less common, the NHS says, and the vast majority of ovarian cysts are non-cancerous.

The NHS says an ovarian cyst usually only causes symptoms if it ruptures, is very large or twists and blocks its own blood supply. Symptoms then can include pelvic pain from a dull heavy feeling to sudden severe sharp pain, bloating, feeling full after eating little, and changes to periods; in early pregnancy the bleeding and pain also overlap with other conditions. Many cysts are found by chance on an early scan done for another reason.

ISUOG notes in its heterotopic pregnancy leaflet that a haemorrhagic corpus luteal cyst can look and feel like an ectopic or heterotopic pregnancy, which is one reason scans in early pregnancy are interpreted by experienced staff and sometimes repeated.

The RCOG says simple ovarian cysts are often found on the scan during pregnancy and most disappear as pregnancy progresses. If a cyst is large or complex you may be offered further scans during pregnancy and a scan after birth. The NHS Borders guideline says ovarian cysts may be found in up to 5% of pregnancies and are rarely malignant, that management of simple cysts less than 5 cm should be conservative with no follow-up, and that corpus luteum cysts usually resolve spontaneously by about the 8th week of pregnancy. It also notes that most adnexal masses in pregnancy resolve or can be managed conservatively.

Two practical points in that guideline: MRI can help in the second and third trimesters when ultrasound is limited, and the blood marker CA 125 is not valid in pregnancy because mild elevation can be normal. A clinician may therefore explain results differently from how they would outside pregnancy.

Twisting of an ovary (torsion) is a clinical emergency that needs rapid diagnosis, according to the NHS Borders guideline. It says torsion rates in pregnancy are 1 to 3%, that the risk reduces with increasing gestation and is unusual after 20 weeks, and that it usually presents with abdominal or pelvic pain, nausea and vomiting, though symptoms can be non-specific. A normal ultrasound does not exclude torsion, and the decision to operate may be made on clinical grounds if symptoms are severe.

The NHS says to ask for an urgent GP appointment or get help from NHS 111 if you have sudden, severe pelvic pain, or tummy pain with nausea or vomiting. If you are pregnant and have severe tummy pain, bleeding with shoulder pain, or feel faint, the NHS pregnancy bleeding page says to call 999. Do not wait to see whether severe pain passes.

Most of the time no treatment is needed. The RCOG says an operation during pregnancy is only recommended for pain thought to be due to the cyst or, very rarely, if cancer is suspected. The NHS Borders guideline says surgery in pregnancy should be performed only for an acute abdomen or high suspicion of malignancy, and that for cysts over 5 cm a follow-up scan should be offered after 6 weeks gestation. It also notes that surgical resection of a corpus luteum cyst may result in miscarriage, and that where intervention is needed because of symptoms it should be delayed until 14 to 16 weeks where possible. The decision is always individual and is for the clinical team to explain; do not stop or start any medicine without their advice.

The NHS says a follow-up scan is often used to confirm a cyst has gone, and the RCOG and NHS both say that the vast majority of cysts are benign. Policies vary between units and between the UK nations.