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Confusing follicles and eggs

Posted Oct 13 2009 10:01pm
Most women are still unclear about the difference between follicles and eggs.

The egg develops within a follicle in the ovary. This follicle ( derived from the Latin word for nest) is a thin-walled structure containing fluid, with the egg attached to the wall. Usually, only one follicle develops per month. This follicular growth can be monitored by vaginal ultrasound scanning.

The follicle appears as a black fluid-filled bubble on the screen, and can be seen when it is about 7 to 8 mm in size. It grows at about 1 to 2 mm per day, and is ready for ovulation when it measures 18 to 25 millimeters in diameter. Following ovulation, the follicle usually disappears from the scan picture completely and this is the best evidence of ovulation.

The egg, on the other hand, is a microscopic structure, which cannot be seen on scanning. For most women, it is true that the follicle does contain an egg. However, this is not always true.

Thus, just because you have follicles in the ovary does not always mean your eggs are of good enough quality to give you a baby. In order to get more information on your egg quality, the doctor will need to perform biochemical tests. These are of two kinds.

The first kind of blood tests check your ovarian quality - what is called ovarian reserve. These include tests such as the level of FSH, AMH and estradiol. These give the doctor an idea about how many eggs are left in your ovary. The correlation between the quality of eggs in the ovary and their quality is usually good. This means that if your ovarian reserve is poor ( you have few eggs left) , this also usually indicates that the quality of your eggs is likely to be poor.

The second kind of test allows the doctor to judge when the follicle is mature and gives an rough measure of the quality of the egg in an individual cycle. The follicle is lined by granulosa cells which produce the hormone estrogen. As the follicle matures, it produces increasing quantities of estrogen. Rising levels suggest that the granulosa cells are of good quality , thus indirectly allowing the doctor to conclude that the egg is of good quality. A mature follicle produces about 200 - 300 pg/ml of estrogen.

However, a direct assessment of the quality of the egg ( the oocyte ) can only be made when doing IVF. At this time we aspirate the follicle and check the retrieved oocyte under the microscope. Unfortunately, it's not easy to judge the quality of the oocyte under the microscope, because most eggs are bland spheres with no distinguishing characteristics. In the final analysis, a good egg is one which will fertilise and become a good embryo !
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