In Vitro Fertilization (IVF) is a process where an egg and sperm are combined outside the body in a laboratory setting. The injection phase, medically termed Ovarian Stimulation, is designed to maximize the number of mature eggs available for retrieval. While a natural cycle usually results in one egg, these daily hormone injections encourage the ovaries to develop multiple follicles simultaneously. Successful stimulation precedes the egg retrieval procedure and is a time-sensitive part of the IVF process.
The Preparatory Phase Leading Up to Injections
Before primary stimulation injections begin, a preparatory phase optimizes the body’s response to fertility drugs. This stage starts with a baseline transvaginal ultrasound and blood work to ensure the ovaries are “quiet,” meaning there are no large cysts and hormone levels are suppressed. This initial screening confirms the body is ready to begin a controlled cycle.
Many protocols use oral contraceptive pills or estrogen priming in the cycle immediately preceding stimulation. The purpose of this step is to synchronize the growth of follicles, preventing one from becoming dominant too early. By ensuring all potential follicles start at a similar size, the medical team aims for a more uniform response to subsequent stimulation medications. This preparation maximizes the number of eggs that will mature once the main injections are introduced.
Initiating Ovarian Stimulation: The Typical Start Date
For most modern IVF cycles, the start of daily stimulation injections is tied directly to the menstrual cycle. The primary injections, which contain gonadotropins like Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), typically begin on Cycle Day 2 or Cycle Day 3 of a full menstrual bleed. This timing is chosen because the body’s natural FSH levels are low, allowing the administered hormones to take over control of follicle development effectively.
Starting the injections early optimizes the recruitment of the cohort of follicles naturally available that month. Introducing high doses of gonadotropins at this time rescues many smaller follicles that would otherwise naturally die off. This controlled hyperstimulation encourages these follicles to grow large enough to yield a mature egg at retrieval. The precise start day is confirmed after baseline blood tests and ultrasound demonstrate that the ovaries are ready.
Navigating the Injection Phase: Monitoring and Duration
Once stimulation injections begin, this phase typically lasts between 8 and 14 days, though the exact duration is highly individualized. Frequent monitoring appointments are required during this time to track follicle growth and check hormone levels. These appointments involve transvaginal ultrasounds to measure the diameter of developing follicles and the thickness of the uterine lining.
Blood tests are performed alongside the scans, primarily to measure estradiol, a form of estrogen that increases as the follicles mature. The results from both the ultrasound and the blood work are used to make necessary, often daily, adjustments to the dosage of the injectable medications. The goal is to safely guide the follicles to a mature size, usually between 18 and 20 millimeters. This close surveillance prevents premature ovulation and ensures the eggs are ready for retrieval 34 to 36 hours after a final “trigger” injection.
How Treatment Protocols Affect Timing
While core stimulation injections usually start on Cycle Day 2 or 3, the overall timing varies significantly based on the treatment protocol chosen. The two most common approaches are the Antagonist protocol and the Long Agonist protocol. The choice of protocol is decided by the fertility specialist based on individual factors like ovarian reserve and medical history.
Antagonist Protocol
The Antagonist protocol is often favored for its shorter timeline, starting main stimulation injections immediately on Day 2 or 3. Suppression of the natural cycle is achieved by introducing a GnRH antagonist injection only after several days of stimulation, typically around Day 5 or 6. This prevents premature ovulation.
Long Agonist Protocol
In contrast, the Long Agonist protocol requires injections to begin much earlier in the preceding cycle, often starting on Day 21. These initial injections, a GnRH agonist, fully suppress the body’s natural hormone production, essentially creating a blank slate. This down-regulation process can take 10 to 14 days before the primary stimulation injections are introduced, making the overall timeline significantly longer than the Antagonist approach.