Written by Dr Sarah Merritt
You may have heard or read the term corpus luteum either during your scan or on your scan report.
The corpus luteum (CL) is a small area on the ovary which is left behind after the egg is released. It is formed from the follicle in which the egg was kept during its maturation and growth and then it is released. As soon as the egg is released the corpus luteum forms and plays a vital role in conception and then its support of the pregnancy in the first and early second trimester.
When women are born, they are born with all the eggs they will ever have. The numbers of eggs are less by the time of the puberty and the onset of periods. During this time until menopause a woman’s body will repeatedly prepare itself for pregnancy, this is the menstrual cycle. The CL will form regardless of a pregnancy or not.
The menstrual cycle comprises of changes in the womb and changes in the ovaries under the control of changing hormone levels released from an area in the brain called the pituitary. The changes in the ovary are called the follicular phase where the egg or eggs which will be released from the ovary are growing and maturing. After ovulation the phase in the ovary is called the luteal phase, which usually lasts about 2 weeks, it is during this time that the CL forms and is functional.
Once the egg is released the corpus luteum will release progesterone and if the egg is not fertilised with sperm the progesterone levels will fall. It is then this drop in progesterone levels which will then cause the lining of the womb to shed and for the period to occur. The CL will collapse and regress, allowing another follicle or follicles to mature and release an egg in the next cycle.
If the egg is fertilised and the pregnancy is ongoing, then progesterone is continuously released from the CL. This makes the CL important in early pregnancy, as the progesterone released helps to support a pregnancy until which the placenta takes over in producing hormones. For women undergoing fertility treatment where they may not have a CL, progesterone is given commonly in the form of a pessary inserted into the vagina or as an injection. This is usually until the nuchal scan or as directed by their fertility team.
On scan it is usually seen as a small cyst of a couple of centimetres big. It can be seen as a small black area on the ovary which has a bright ring of colour around it. It can also be more ‘solid’ (where it looks like it has a light shade of grey) in appearance with the same colour ring around it. Colour Doppler can be put onto the ovary in early pregnancy but not the pregnancy in the womb.
The ovary which contains the CL can typically be quite tender and cause pain. This is normal. Some women find it very painful and will often have a scan in early pregnancy to ensure that the pain is not caused by an ectopic pregnancy. Pain caused by the CL usually settles with simple pain relief such as paracetamol.