Ovaries

The ovaries produce and release eggs (oocytes) into the female reproductive tract at the mid-point of each menstrual cycle. They also produce the female hormones oestrogen and progesterone and androgens.

Alternative names

Where are the ovaries?

Artwork of the female reproductive system, showing the location of the ovaries.

The ovaries are a key part of the female reproductive system. Each woman has two ovaries. They are oval in shape, about four centimetres long and lie on either side of the womb (uterus) against the wall of the pelvis. They are held in place by ligaments attached to the womb but are not directly attached to the rest of the female reproductive tract, e.g. the fallopian tubes.

What do the ovaries do?

The ovaries have two main reproductive functions in the body. They produce oocytes (eggs) for fertilisation and they produce the reproductive hormones, oestrogen, progesterone and androgens. The function of the ovaries is controlled by gonadotrophin-releasing hormone (GnRH) released from the hypothalamus which in turn stimulates the pituitary gland to produce luteinising hormone (LH) and follicle stimulating hormone (FSH). These hormones are carried in the bloodstream to the ovary to regulate the menstrual cycle.

The ovaries release an egg (oocyte) at the midway point of each menstrual cycle. Usually, only a single oocyte from one ovary is released during each menstrual cycle, known as ovulation. A female baby is born with all the eggs that she will ever have. This is estimated to be around two million, but by the time a girl reaches puberty, this number has decreased to about 400,000. From puberty to the menopause, only about 300 – 400 eggs will be released through ovulation.

The ovarian phases of a 28-day menstrual cycle. Ovulation occurs mid-cycle.

In the ovary, all eggs are initially enclosed in a single layer of cells known as a follicle, which supports the egg. During the follicular phase (first part of the menstrual cycle), one or two ovarian follicles grow due to the action of FSH. As the follicle grows it produces oestradiol. As oestradiol levels rise this induces the hypothalamus and pituitary gland to make high levels of LH (and some FSH) at the midpoint of the cycle to induce ovulation. During ovulation, the egg is released from the follicle in the ovary into the fallopian tube.

Once the egg has been released at ovulation, the empty follicle that remains becomes the corpus luteum (CL). The CL produces the hormones progesterone (in a higher amount) and oestrogen (in a smaller amount). These hormones prepare the lining of the uterus for a potential pregnancy (in the event that the released egg is fertilised by sperm in the female reproductive tract). If the released egg is not fertilised and pregnancy does not occur during a menstrual cycle, the corpus luteum breaks down and the secretion of oestrogen and progesterone stops. Due to the fall in levels of progesterone, the lining of the womb starts to fall away and is lost from the body through menstruation, or a ‘period’. Menstruation usually lasts around 3 – 5 days. Day 1 of menstruation signals the start of a new menstrual cycle.

The menopause refers to the ending of a woman’s reproductive years following her last menstruation, and is around 51 years of age. This is caused by loss of the remaining follicles in the ovary. When there are no more follicles (which each contain an egg), the ovary also no longer makes the hormones oestrogen and progesterone, which regulate the menstrual cycle. As a result, the occurrence of menstrual cycles and monthly periods ceases.

What hormones do the ovaries produce?

The major hormones secreted by the ovaries are oestrogen and progesterone, both important hormones in the menstrual cycle. Oestrogen production dominates in the first half of the menstrual cycle before ovulation, and progesterone production dominates during the second half of the menstrual cycle after the corpus luteum has formed. Both hormones are important in preparing the lining of the womb for pregnancy and the implantation of a fertilised egg, or . If conception occurs during a menstrual cycle, the corpus luteum does not lose its ability to function and continues to secrete oestrogen and progesterone, allowing the embryo to implant in the lining of the womb and form a placenta. The ovaries also make small amounts of androgens (male hormones).

What could go wrong with the ovaries?

Any medical conditions that stop the ovaries from functioning properly can decrease a woman’s fertility. The ovaries naturally stop functioning at the time of menopause. This occurs in most women around the age of 51 years. If this happens earlier, e.g. before the age of 40 years, it is termed ‘premature ovarian insufficiency’ (previously known as premature ovarian failure). Hormone Replacement therapy (HRT) is the most common treatment to replace the effects of the missing ovarian hormones).

Any abnormality that causes a loss of normal development of the ovaries, such as Turner syndrome, can result in the ovaries not functioning correctly and can result in the loss of a woman’s fertility. The ovaries can also be damaged by treatments for other conditions, particularly chemotherapy or radiotherapy for cancer treatment.

The most common disorder of the ovaries is polycystic ovary syndrome, which affects 8–13% of women of childbearing age. In a polycystic ovary, the follicles mature to a certain stage, but then stop growing and fail to release an egg. These stunted follicles can appear as cysts in the ovaries on an ultrasound scan (termed ‘polycystic ovarian morphology’). Affected women may have symptoms of excess male hormones (hyperandrogenism), such as excess hair growth (hirsutism) or acne, or not ovulating (anovulation) leading to irregular (oligomenorrhoea) or absent (amenorrhoea) periods. PCOS can also be associated with a high body mass index (BMI), and with insulin not working as efficiently (insulin resistance) leading to an increased risk of type 2 diabetes.

If a woman stops having menstrual periods during her reproductive years, this condition is called amenorrhoea. It can be caused by a number of factors. These include hypothalamic amenorrhea, which may be caused by having insufficient energy availability due to decreased nutritional intake leading to low body weight, excessive exercise, often in combination with psychological stress. Disorders of the pituitary gland, such as hypopituitarism caused by pituitary tumours, or excess prolactin, can also affect ovarian function by leading to the lack of hormones normally released from the pituitary gland (LH / FSH), which will thus reduce stimulation of ovaries to grow follicles or make hormones.