During an IVF cycle, your doctor uses medications to help you grow grow more eggs. The reason we need to superovulate is because we need more eggs to get more embryos, to help maximise your chances of success.
In order to superovulate you, we rely primarily on gonadotropin injections . These injectibles are called menotropins ( HMG, or human menopausal gonadotropins) - and these maybe urinary products ( such as Menogon or Repronex) , or recombinant gonadotropins ( such as Follistim or Gonal-F). They all contain FSH ( with or without the other gonadotropin, LH) . They all act exactly in the same way - they stimulate the follicles in your ovaries to grow.
Superovulation usually starts on Day 2 or 3. The reason for this is simple. This is because your body starts selecting which follicles will start to grow each month from Day 1 - 3 of each menstrual cycle. This is called the phase of follicular recruitment, at which time about 30-40 resting follicles are selected for growth. In the natural cycle, this is a result of a boost in the production of FSH ( = follicle stimulating hormone) by the pituitary. The gondaotropin injections we use for superovulation contain exactly the same FSH ! Of the selected follicles, only one becomes mature or dominant, while the other recruited follicles are destined to die naturally every cycle , in a process is called atresia. The gondaotropins injectibles allow the doctor to rescue these follicles, so that they do not die, but continue to grow. This is why IVF meds are safe and will not cause you to run out of eggs or become menopausal early - they just help us to save follicles which would have died anyways.
Interestingly, with the move towards single embryo transfer ( SET ), some clinics today do not use these injections at all ! They use gentler stimulation protocols with ovulation induction drugs such as clomid; while others use a natural cycle, and do IVF with only a single egg. While this does work, the success rate with mini-IVF is lesser, because there are fewer eggs.
Ovulation induction drugs such as clomid or letrozole act in exactly the same way, by pushing your own pituitary to produce more FSH. Thei major benefit is that they can be taken orally; and are much less expensive. However, they are less powerful, and help you to grow fewer mature eggs.
That sounds simple enough. All the doctor needs to do is: inject gonadotropins from Day 1 ( or 2 or 3); grow lots of follicles ; and collect the eggs from the follicles when they are mature ! So what's the big deal ?
The problem is that not all women will respond to the meds in the same way. There is no standard formula which a doctor can use to decide which dose is right for a particular patient ; and part of the secret sauce of being a good IVF doctor is that we are much better at selecting the right dose of injections to ensure you grow the right number of eggs - not too many ( because of the risk of OHSS); and not too few ( as this would reduce your chances of success, because you would have fewer embryos).
So how do we select the right dose ? A lot of this does depend upon experience, and there are many factors we weigh when choosing the dose. We consider many variables, including: your age; your ovarian reserve ( as measured by your antral follicle count and your AMH level); and your past response to medications.
However, the fact still remains that biological responses can vary; and we do need to carefully monitor your superovulation, to make sure you are growing as expected. This is why the scan which we do 6 days after we start your superovulation is a moment of truth scan ! Have you responded well ? Or do we need to tweak the dose ? While most women will do well, some will be quite challenging. Thus, they may grow only a few follicles; or the response may not be uniform, as a result of which some follicles are large; others remain small; and some form cysts. This can be quite perplexing, because it then becomes much harder to judge when the follicles are mature enough for egg retrieval.
We do have certain rules of thumb which offer a rough guideline. Thus, for most women the starting dose is 225 IU per day. For older women, we will increase it; and for women with PCOD, we reduce this.
Just growing follicles is not enough ! We need to ensure that we have a group ( cohort) of mature follicles , so that we can retrieve many mature eggs for you. Egg collection needs to be timed precisely - we don't want to go in too early and get immature eggs, which will not fertilise. Neither do we want to wait too long and allow the follicles to rupture. This is where the HCG trigger and the downregulation meds come into play and I will discuss these in another article.