Ovarian Stimulation in IVF: Protocols, Timeline, and Safety

Key Takeaways

Ovarian stimulation encourages a cohort of follicles to develop using tailored gonadotropin doses over about 8 to 14 days. Protocol choice depends on ovarian reserve, age, and health history. The aim is mature eggs with the lowest workable risk of ovarian hyperstimulation syndrome (OHSS). GnRH antagonist protocols, with an agonist trigger when needed, recruit follicles with a lower hyperstimulation risk than older long-agonist plans.

Key evidence: 2025 ESHRE Guideline: Ovarian Stimulation for IVF/ICSI · ASRM Practice Committee: Prevention of Moderate and Severe OHSS · Cochrane Review: GnRH Antagonists vs Agonists in IVF

Understanding Ovarian Stimulation in IVF

Ovarian stimulation is the first medical stage of an in vitro fertilization (IVF) cycle. Its purpose is to help several ovarian follicles, the fluid-filled sacs that each hold an egg, grow together within a single cycle.

In a natural cycle, the pituitary gland releases follicle-stimulating hormone (FSH) in an amount that lets only one dominant follicle reach ovulation. The other follicles recruited in that monthly wave shrink away on their own, a process called atresia. In IVF, daily gonadotropin injections support that same monthly group, so the embryology team can collect, fertilize and assess several eggs.

Several eggs give the laboratory a group to fertilize and grow to the blastocyst stage. Not every retrieved egg is mature, and not every fertilized egg becomes a viable embryo. A reasonable number of mature eggs therefore improves the cumulative chance of a live birth across fresh and frozen transfers.

Medication needs vary widely from person to person. Gonadotropin doses are set for the individual and adjusted during the cycle, not given by a fixed formula.

Dr. Aksoy’s Approach:
Dr. Aksoy uses a GnRH antagonist protocol as the first-line plan for most patients, especially with PCOS or a predicted high response. The aim is a well-matured cohort rather than the highest possible egg count: doses stay proportionate, and for high responders an agonist trigger with freeze-all is used to reduce late-onset OHSS.

Why Stimulation Protocols Differ

No single protocol suits every patient. A regimen that gives one person a safe, moderate response may give another too few follicles, or far too many.

To tailor a plan, fertility specialists look at the markers and patient characteristics described in the 2025 ESHRE Guideline on Ovarian Stimulation for IVF/ICSI:

• Ovarian reserve markers: Blood anti-Müllerian hormone (AMH) and the baseline antral follicle count (AFC), counted on a transvaginal ultrasound.
• Age and reproductive history: Age relates to how likely eggs are to be chromosomally normal and to how the ovaries respond to gonadotropins.
• Underlying diagnosis: With polycystic ovary syndrome (PCOS) the follicles are more sensitive, so starting doses are usually cautious. With a low reserve, different dosing strategies may be considered.
• Body mass index (BMI): It affects how medication is distributed and cleared.
• Response in earlier cycles: Previous IVF attempts help guide starting doses and the choice of trigger.

Common Protocol Types

1. GnRH Antagonist Protocol:
   The first-line protocol that international guidelines recommend for most IVF patients. Gonadotropin injections start on day 2 or 3 of the menstrual cycle. A GnRH antagonist (such as cetrorelix or ganirelix) is added to prevent a premature luteinizing hormone (LH) surge. In a fixed protocol it starts on day 5 or 6 of stimulation; in a flexible protocol it starts when the lead follicles reach 12–14 mm. The 2025 ESHRE update conditionally recommends the fixed approach over the flexible one. A Cochrane systematic review found live birth rates comparable to agonist regimens, with less ovarian hyperstimulation.

2. Long GnRH Agonist Protocol:
   A GnRH agonist is started in the mid-luteal phase of the previous cycle to switch the pituitary off (down-regulation) before gonadotropins begin. It is used less often today because treatment takes longer and the OHSS risk is higher. It remains an option in selected situations, such as some cases of endometriosis or adenomyosis, or when the cycle must be scheduled precisely.

3. Mild or Low-Dose Stimulation:
   Lower gonadotropin doses are used, sometimes with an oral drug such as letrozole. It is considered for some women expected to respond poorly, when higher doses do not yield more mature eggs, or for patients who want a lighter regimen.

The Stimulation Timeline: Typical Stages

An ovarian stimulation phase generally lasts between 8 and 14 days before egg retrieval.

Cycle day 2–3       Stimulation day 1      Stimulation day 5–6 on    Trigger day            34–36 h later
[ Baseline scan ] > [ Gonadotropins ] ───> [ Antagonist added ] ───> [ Maturation trigger ] > [ Egg retrieval ]
  AFC & hormones      Daily injections       Monitoring scans          hCG or agonist         Short procedure

1. Baseline Assessment (Cycle Days 2–3)

Before any medication, a transvaginal ultrasound confirms that the ovaries are quiet, checks for leftover functional cysts and counts the resting antral follicles. Baseline estradiol (E2) and progesterone are measured when indicated.

2. Gonadotropin Injections (From Stimulation Day 1 to the Trigger)

Daily subcutaneous injections of follicle-stimulating hormone (recombinant FSH or human menopausal gonadotropin, hMG) begin and continue until the trigger. They are given once a day in the abdomen, usually at the same time each evening.

3. Monitoring Visits (From About Stimulation Day 5–6)

From around day 5 or 6 of stimulation, clinic visits take place every 2 to 3 days. Transvaginal ultrasound tracks follicle growth, which averages roughly 1.5 to 2 mm a day, and blood tests follow estradiol. Doses may be adjusted according to how the follicles grow.

4. Preventing an Early LH Surge

In an antagonist protocol, the antagonist is added to FSH on the planned day (often day 5 or 6 of stimulation) or when the follicles reach the size threshold. It stops the LH surge that would release the eggs too early.

5. Final Maturation Trigger

When enough lead follicles reach mature size (commonly 17–18 mm or more, read in the context of the whole cohort), the trigger injection is timed. It completes the final maturation of the eggs. Depending on the individual hyperstimulation risk, the trigger may be human chorionic gonadotropin (hCG), a GnRH agonist (such as triptorelin) or a dual trigger that combines both.

6. Timed Egg Retrieval

Eggs are collected about 34 to 36 hours after the trigger, before spontaneous ovulation. The procedure is done transvaginally under ultrasound guidance with sedation or anaesthesia and usually takes 15 to 20 minutes.

Oocyte Yield: Fresh Outcomes, Cumulative Potential, and Safety

How many eggs are collected affects a fresh transfer differently from the cumulative result across all later frozen transfers.

Large early registry analyses, such as Sunkara et al. (2011), showed that in fresh transfer cycles the live birth rate rises with the number of eggs up to about 15. Beyond that, fresh outcomes level off while hyperstimulation risk keeps rising.

In contrast, analyses of cumulative live birth—fresh transfers plus later frozen transfers from the same retrieval—show that the chance can keep rising with higher oocyte numbers. In a 2009–2015 ANZARD cohort of 221,221 autologous aspiration cycles, the extra gain per additional egg was larger in younger women (Law et al., 2019). A 2025 pooled analysis of 1,746 follitropin-delta cycles found cumulative live birth still rising to about 21–25 eggs; the continued gain beyond 15 eggs was seen in women aged 38 and over, not in younger women (Lobo et al., 2025).

A high egg count has trade-offs. In some high-response cycles, progesterone rises early in the late follicular phase and may put the lining out of step with the developing embryo. This is one reason clinicians may discuss freezing all embryos (ESHRE 2025 guideline).

Side Effects and Ovarian Hyperstimulation Syndrome (OHSS)

Most patients notice mild, temporary symptoms during stimulation as the ovaries enlarge:

• Mild lower abdominal fullness or pelvic pressure
• Moderate bloating
• Breast tenderness and hormonal mood fluctuations
• Minor local redness or bruising at injection sites

Preventing and Managing OHSS

Ovarian hyperstimulation syndrome (OHSS) occurs when the ovaries over-respond and release substances such as vascular endothelial growth factor (VEGF) that make blood vessels leak. Moderate-to-severe OHSS used to affect roughly 1% to 5% of IVF cycles; modern prevention has lowered this considerably (ASRM Practice Committee Guideline):

• GnRH Agonist Trigger: In women at risk of OHSS, such as those with a high response or PCOS, a GnRH agonist trigger instead of hCG combined with freezing all embryos causes a shorter LH surge and lowers the risk of severe OHSS. The 2025 ESHRE update does not recommend an agonist trigger in the general population when a fresh transfer is planned.
• Elective Freeze-All Strategy: When all blastocysts are frozen and the transfer is postponed, the body is not exposed to hCG from an early pregnancy, the main driver of severe late-onset OHSS.
• Dopamine Agonists (Cabergoline): Prescribed selectively around the time of retrieval for patients at high risk, to reduce vascular leakage.

Warning Signs Requiring Prompt Medical Review

Mild bloating is expected. Contact your clinical team promptly if you have:

• Rapidly worsening abdominal distension or severe pain
• Persistent nausea or vomiting preventing fluid intake
• Noticeable shortness of breath or discomfort when breathing deeply
• Sudden weight gain exceeding 1 kg (2.2 lbs) in 24 hours
• Significantly reduced or dark, concentrated urination

Practical Considerations During Stimulation

1. Follow Injection Instructions: Give each injection in the daily window your clinic recommends, so hormone levels stay steady.
2. Check Storage Requirements: Read the packaging. Some gonadotropins need the refrigerator (2°C to 8°C); others can be kept at room temperature after first use.
3. Adjust Physical Activity: As the ovaries enlarge, avoid high-impact workouts, heavy lifting and sudden twisting movements, which lower the risk of ovarian torsion.
4. Drink Well and Eat Balanced Meals: Plenty of fluids and enough dietary fiber help digestion and comfort.
5. Stay in Touch with Your Clinic: Call your coordinator whenever you have a question. Changing doses after ultrasound and blood results is a normal part of individualized care.

Related Reading

• IVF Protocols and Medications: How Doctors Choose a Plan
• How Many Eggs Are Usually Enough for IVF?
• Egg Retrieval Pain Control: Sedation, Anaesthesia, and What to Expect

Frequently Asked Questions for Dr. Aksoy

Can I drink alcohol during ovarian stimulation?

Patients having IVF sometimes ask me: I have started the injections — would a glass of wine in the evening be a problem?

My advice is not to drink alcohol during ovarian stimulation if you can avoid it. It matters to understand why.

We know that drinking larger amounts of alcohol can harm fertility. By contrast, how much an occasional small amount during stimulation affects egg quality or IVF success has not been shown with certainty. Study results are not always consistent with one another.

I do not want you to think that one glass of wine has damaged your eggs or that the whole treatment has been wasted.

That does not mean I would specifically recommend alcohol either. We are already in a period when we are trying to achieve a pregnancy. Egg retrieval is also usually done with sedation or anaesthesia, so you need to follow your clinic’s instructions about alcohol before and after that procedure.

After embryo transfer my approach is clearer: I advise against alcohol, because a pregnancy may be starting. No amount of alcohol has been shown to be safe in pregnancy.

What I tell patients is this: not drinking during stimulation is the safest choice. Having had a small amount once is not a reason to panic, or to assume that treatment will fail.

Frequently Asked Questions

Does ovarian stimulation deplete future egg reserve or bring on earlier menopause?

Each menstrual cycle, a group of small resting follicles is recruited naturally. In an unmedicated cycle all but one of them are lost through atresia. Stimulation medicines support the follicles of that same monthly group so they can grow. Current evidence does not show that this draws on future egg reserve or brings menopause forward.

What happens if follicles grow at different rates?

Uneven growth is common. The team follows the whole cohort and times the trigger for when most lead follicles are mature, weighing the number of mature eggs against the risk that the earliest follicles become over-mature.

Can an IVF stimulation cycle be adjusted or cancelled?

Yes. Doses are adjusted to your response. If the response is far below or above the safe range, the team may change the doses or the trigger plan, or occasionally cancel the cycle and restart with a revised protocol.

How are daily fertility injections generally tolerated?

Most patients find the daily injections manageable. They use fine needles into the fat just under the skin of the abdomen, and mild redness or brief stinging usually fades quickly.

Sources

• ESHRE Guideline Group on Ovarian Stimulation for IVF/ICSI. (2026). ESHRE guideline: ovarian stimulation for IVF/ICSI: an update in 2025. Human Reproduction, 41(4), 498-514. DOI
• Practice Committee of the American Society for Reproductive Medicine. (2024). Prevention of moderate and severe ovarian hyperstimulation syndrome: a guideline. Fertility and Sterility, 121(2), 230-245. ASRM Practice Guidance
• Al-Inany, H. G., Youssef, M. A., Ayeleke, R. O., Brown, J., Lam, W. S., & Broekmans, F. J. (2016). Gonadotrophin-releasing hormone antagonists for assisted reproductive technology. Cochrane Database of Systematic Reviews, 4, CD001750. PubMed: 27126581
• Sunkara, S. K., Rittenberg, V., Raine-Fenning, N., Bhattacharya, S., Zamora, J., & Coomarasamy, A. (2011). Association between the number of eggs and live birth in IVF treatment: an analysis of 400 135 treatment cycles. Human Reproduction, 26(7), 1768-1774. PubMed: 21558332
• Law, Y. J., Zhang, N., Venetis, C. A., Chambers, G. M., & Harris, K. (2019). The number of oocytes associated with maximum cumulative live birth rates per aspiration depends on female age: a population study of 221 221 treatment cycles. Human Reproduction, 34(9), 1778-1787. Human Reproduction
• Lobo, R., Santos-Ribeiro, S., Falahati, A., et al. (2025). One-year cumulative live birth rate associated with the number of oocytes in ovarian stimulation with follitropin delta: a pooled analysis of four randomized controlled trials. Human Reproduction, 40(8), 1526-1534. PubMed: 40505136

Dr. Senai Aksoy

Dr. Senai Aksoy studied and trained in France before returning to Turkey, where he was a member of the ICSI team at Sevgi Hospital, Ankara — the country's first ICSI centre (1994-95) — and a co-author on the first Turkish ICSI publications produced in collaboration with the Brussels Van Steirteghem group (Human Reproduction, 1996; PMID 8671323). He helped build the IVF programme at the American Hospital Istanbul and has been running his own fertility practice since 1998.

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The content has been created by Dr. Senai Aksoy and medically approved.