In a woman who ovulates normally, follicle-stimulating hormone (FSH) brings about the growth of a single follicle at the start of every menstrual cycle. Follicles are fluid filled sacs within the ovary, each of which contains an immature egg. Once the follicle reaches a certain size (usually 18mm) a sudden mid-cycle surge of luteinising hormone (LH) causes ovulation (rupture of the follicle, releasing the egg from the ovary). In some women ovulation does not occur regularly, making it difficult to conceive (most commonly in women with polycystic ovarian syndrome or PCOS).
Ovulation induction (the simplest form of fertility treatment) is recommended for women who are not ovulating. Clomid tablets or low dose FSH injections are administered with the aim of stimulating the growth of one or two follicles. Follicle tracking scans are carried out to make sure the treatment is working, i.e. that follicles are, in fact, growing and that not too many follicles are growing (if three follicles grow, a triplet pregnancy could result). Once the follicle is big enough ovulation is triggered with an injection of HCG, (this has the same biological action as LH). Ovulation will take place over the next thirty-six hours and the couple are advised to have intercourse in this period.
Administration of Clomid without follicle tracking scans and a HCG trigger is often inefficient (because follicles may or may not grow and may or may not rupture), and occasionally dangerous because triplet pregnancies can result. Follicle tracking by experienced fertility clinicians is important.
A typical OII cycle with clomid takes place as follows:
- The woman takes Clomid daily for 5 days, starting on cycle day 2 (the second day of her period)
- She attends for a follicle tracking trans-vaginal scan on cycle day 12
- 1 or more further follicle tracking scans may be necessary, depending on how quickly the follicles start growing
- Once the follicle has reached a size of 18mm a hCG injection is given and the couple are advised to have intercourse the same evening and the following day
- A pregnancy test is carried out two weeks later