Bird have you had 21 day bloods done already? How long is your cycle - if you ovulate later then it may be that they have decided to do them on CD26 to get the most realistic responde to see if you are ovulating? CD21 might be too soon?
CD3 bloods will show your hormone levels - i got told that they checked rubella along with your hormone levels post/pre ovulation which would show if you were peri(?) meno pausal etc. and how well your hormones stimulate your ovaries etc....since i did such a poor job explaining here's a bit of cut and paste!!!!
Testing for the woman first looks at whether she is ovulating each month. This can be done by having her chart changes in her morning body temperature, by using an FDA-approved home ovulation test kit (which she can buy at a drug store), or by looking at her cervical mucus, which changes throughout her menstrual cycle. Ovulation also can be checked in her doctor's office with an ultrasound test of the ovaries, or simple blood tests that check hormone levels, like the follicle-stimulating hormone (FSH) test. FSH is produced by the pituitary gland. In women, it helps control the menstrual cycle and the production of eggs by the ovaries. The amount of FSH varies throughout the menstrual cycle and is highest just before an egg is released. The amounts of FSH and other hormones (luteinizing hormone, estrogen, and progesterone) are measured in both a man and a woman to determine why the couple cannot achieve pregnancy. If the woman is ovulating, more testing will need to be done. These tests can include:
an hysterosalpingogram (an x-ray to check if the fallopian tubes are open and to show the shape of the uterus)
a laparoscopy (an exam of the tubes and other female organs for disease)
an endometrial biopsy (an exam of a small shred of the uterine lining to see if monthly changes in it are normal)
Other tests can be done to show whether the sperm and mucus are interacting in the right way, or if the man or woman is forming antibodies that are attacking the sperm and stopping them from getting to the egg.
Ah - that website said it far better than i could!! xx