What the risks are:

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks related to having blood drawn are rare but may include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Special considerations:

In women, estrogen levels rise during the menstrual cycle until there is enough estrogen present to stimulate the release of GnRH and lutenizing hormone (LH).

Progesterone in high concentrations (for example, during the luteal phase of the menstrual cycle or during pregnancy) reduces the response of the pituitary gland to GnRH.