Trigger Shot for IVF and Its Importance in Egg Quality

Trigger shot palys an important role in IVF treatment. The purpose of an IVF trigger shot is to send the eggs into a state of reproductive cell division called meiosis (also known as maturation division), during the 36-hour period preceding ovulation or retrieval.

The timing and efficiency of this process determine the quality of eggs that are retrieved and used in embryo creation and the outcome of the cycle.

When it comes to ovulation induction during the IVF treatment, we should choose and implement the trigger shot with utmost care. It is one of the most important decisions a physician must make as the trigger shot greatly affects the success rate of the IVF cycle.

Let us see how IVF trigger shot works and some common medications used for this purpose.

How the Trigger Shot Works for IVF

Gonadotropin Simulation

The trigger shot in actuality is human chorionic gonadotropin (hCG), a hormone that signals the follicles to rupture and release an egg.

In an ovarian stimulation cycle, gonadotropic hormones such as FSH and LH stimulate the ovaries for egg development in order to achieve pregnancy.

The trigger shot must be administered approximately 36 hours before IVF egg retrieval as the timing is crucial for the success of the entire IVF process. This trigger shot comprises of one of three medications:

1. Urinary derived hCG (hCGu) shot such as Noravel, Pregnyl, and Profasi
2. Recombinant hCG (hCGr) shot such as Ovidrel shot
3. Agonist trigger shot such as Lupron as it provokes a surge in the release of LH upon being injected

Meiosis of Egg Cells

As said earlier, the purpose of the trigger shot in IVF treatment is to send eggs into a reproductive division known as meiosis.

During meiosis, the objective is to decrease the number of chromosomes in the eggs from 46 to 23 prior to egg retrieval or ovulation. Trigger shots expel half of the chromosomes from the egg’s nucleus in a membranous envelopment.

This so-called first polar body comes to lie immediately under the envelopment of the egg in a region known as perivitelline space. Here it seems as if the cells surrounding the egg are removed through microsurgery, but they can also erode on their own.

Microscopic Visualization

Microscopic visualization of the polar body confirms that the eggs are mature, meaning it has gone through meiosis. However, it provides no assurance that there are only 23 chromosomes remaining in the egg’s nucleus.

If exactly 23 chromosomes are not present in the nucleus of the fertilized egg, it will be unable to create a euploid embryo (one with 46 chromosomes).

Note: An aneuploid embryo is one that has an irregular number of chromosomes (more or less than 46) and is incapable of creating a healthy baby.

Hence, the dosage, method, and administration of the trigger shot affect the potential to yield mature quality eggs. The said trigger shot factors also represent a rate-limiting step in ovulation induction specifically for the IVF treatment cycle.

Types of Trigger Shot: Urinary vs. Recombinant hCG or Ovidrel

Previously, the standard method used to initiate the IVF trigger shot was through the administration of 10,000 units of hCGu. More recently, Ovidrel trigger shot, a recombinant form of hCG, has been introduced given in in 250 mcg doses.

Ovidrel trigger shot is an injectable medication that can be used by itself or in a combination with other fertility drugs. Apart from IVF, it can also be used to increase the chances of success during timed sexual intercourse, and an intrauterine insemination (IUI) cycle.

Using Trigger Shot in a Timed Intercourse

A trigger shot is often utilized for timed intercourse or SMEP. Once the eggs reach the optimal development stage, a trigger shot is given, that prompts ovulation within 36–42 hours.

After the trigger shot is administered, the couple is instructed to have intercourse either daily or every other day for three days.

However, if you are taking the shot as a part of your ovulation induction protocol, your doctor will guide you on when and how to take it synchronized to other medications and procedures.

Using Trigger Shot for IUI

A trigger shot can also be used for Intrauterine Insemination or IUI cycle when the follicle is a minimum of 20 mm with the uterine lining being at least 7 mm. Typically, Ovidrel costs around $100–$200 depending on the pharmacy you are visiting.

Shortfall

Experience suggests that 250 mcg is not equivalent in biological potency to 10,000 units of hCG. The potency is probably only 50% to 70% as compared to 10,000 doses of hCGu. This might not be enough to fully promote meiosis, especially in cases where a woman has numerous follicles.

For this reason, it’s firmly believed that the dosage should be doubled to 500 mcg when using hCGr for “trigger shot.” At this dosage, it will have an effect equivalent to 10,000 units of hCGu. Having said this, it is unnecessary to supplant hCGu with hCGr since the latter is more expensive and is no more potent than the former.

Dosage of hCG Trigger Shot

Many clinicians, when faced with risks of OHSS developing, deliberately elect to reduce the dosage of hCG administered as trigger shot. Doing so significantly reduces the possibility of the risk of developing critical OHSS.

However, a low dose of hCG (5,000 units) is likely to be inadequate to optimize the efficiency of meiosis, especially in the case where there are numerous follicles.

Use of hCG Trigger Shot vs. Agonist

It has been suggested that the use of an agonist, such as Lupron, as a trigger shot in women at risk of developing severe ovarian hyperstimulation syndrome, could potentially reduce the risk of the condition becoming critical.

This is why many prefer to trigger meiosis with Lupron rather than the use of hCG. Lupron promptly causes the woman’s pituitary gland to expunge a large amount of LH over a short period of time. It’s this surge that triggers meiosis. However, there’s a problem with this approach.

It’s hard to predict how much LH will be released by the pituitary gland of a patient receiving Lupron. Especially if the patient was downregulated using an agonist or where an antagonist was used to block pituitary LH release. For this reason, it is preferable to use hCGu trigger shot, even with a mild risk of ovarian hyperstimulation.

The Timing of The Trigger Shot During IVF

This must coincide with most of the ovarian follicles being greater than 15 mm in diameter and with several follicles having reached 18 to 22 mm. The follicles grow an average of 2 mm per day during the later stages of stimulation.

Follicles larger than 22 mm will usually harbor overdeveloped eggs which in turn are most likely to fail to produce good quality eggs. Follicles less than 15 mm will also harbor underdeveloped eggs that are most likely to be incompetent following the trigger.

Conclusion

The protocol selected for ovarian stimulation is one of the most important aspects of IVF treatment because optimal egg maturation depends on prior egg development. Unless the follicles and eggs can develop optimally, no trigger shot might be capable of yielding good quality eggs.

We hope the above discussion helped you understand how trigger shots work and will help you proceed with your trigger shot more easily.

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