IVF Success Rates Statistics

1. Embryologist Nikolova, in one of his videos Dr. Stamenov talks about IVF statistics and how they should be interpreted correctly. What does “success rate” mean in IVF procedures?
Success rate means achieving pregnancy and the birth of a child following an IVF procedure. It is an indicator of the effectiveness of IVF treatment and is usually expressed as a percentage.

There is no universal methodology for calculating IVF success rates in a way that allows direct comparison between clinics. Success can be calculated in several different ways:

• Success rate per initiated/financed cycle – the percentage of pregnancies per initiated IVF cycle (out of 100 started IVF procedures, how many result in pregnancy). This includes cycles that do not reach embryo transfer, frozen eggs, embryos, or embryos that stop developing.
• Success rate per embryo transfer – the percentage of pregnancies from all embryo transfers performed (if 100 women undergo embryo transfer, how many become pregnant from that specific transfer, without tracking remaining embryos).
• Cumulative success rate from one or several cycles – if we follow 100 patients and all their fresh and frozen transfers, how many eventually achieve pregnancy.

2. Patients often search for and compare overall success rates when choosing a clinic. How should IVF statistics be interpreted properly?
The term “overall success rate” is not appropriate for comparing clinics because all patients are grouped together regardless of age, diagnosis, or case complexity.

To make meaningful comparisons, data must be grouped and analyzed correctly. Clinics should only be compared using similar patient groups—for example, patients within the same age range. If one clinic treats a larger proportion of older patients, it will naturally handle more complex infertility cases, which logically lowers the overall success rate.

It is also not sufficient to simply count how many patients underwent ovarian puncture in one calendar year and how many became pregnant or gave birth during that same year. Transfers, pregnancies, and births may occur months or even years later due to frozen embryo transfers, natural cycle egg collections, and other factors.

This also raises an important ethical question: should clinics compete for high success rates by refusing difficult patients with lower chances of pregnancy? Where should patients go when they have unexplained infertility, poor egg retrieval outcomes, or embryos that repeatedly stop developing?

These patients are aware that their prognosis may be poor, yet they still wish to try every possible option. While not every case can be successful, helping even some of these couples changes lives.

At Nadezhda Hospital, the goal is to offer every couple the best possible care by examining each case comprehensively, applying different stimulation protocols, performing embryo biopsies and genetic testing, and doing everything possible to achieve pregnancy.

According to Stefka Nikolova, patients should choose a clinic with strong laboratories, experienced specialists, and a medical team they trust—one that communicates clearly and works together with them toward achieving pregnancy.

3. Many patients rely on the statistics published by the Bulgarian Center for Assisted Reproduction (CAR). What should we know about them?
Patients often review the reports of the Bulgarian Center for Assisted Reproduction (CAR), but CAR statistics are calculated differently.

The reports mainly show:

• how many procedures were financed during a calendar year;
• how many pregnancies and births were registered during that same year.

However, pregnancies recorded in a given year may result from procedures financed up to four years earlier.

If a clinic finances more patients during a year but fewer reach embryo transfer immediately, the percentage decreases, even though this does not reflect the actual quality of care.

For meaningful comparison, success rates should be measured per transfer or per patient within comparable patient groups.

Another important factor is that a CAR approval order is valid for one year and may span two calendar years. One approval may include:

• one stimulated IVF cycle;
• or up to four natural-cycle punctures.

Very few clinics work extensively with natural-cycle IVF patients. In addition, oncology patients preserving eggs through cryopreservation are also included in the total number of financed procedures.

4. Does the number of patients treated annually affect success rates
Absolutely.

Small clinics with very few annual cycles can easily report 50–100% success rates, but statistically these numbers are not representative.

For example:

• if a clinic treats two patients in one year and both become pregnant, it reports a 100% success rate;
• if one succeeds, the clinic still reports 50%.

When a clinic treats 300–1000 patients annually, the situation changes dramatically because the patient population becomes much more diverse in terms of age, diagnoses, and complexity. At that point, success rates reflect real clinical practice much more accurately.

5. Which factors influence IVF success rates?
Success depends on many different factors, which can generally be divided into two groups:

Patient-related factors

• age of both partners;
• causes of infertility;
• quality of genetic material;
• condition of the uterus and endometrium;
• genetic causes;
• immunological causes.

Clinic-related factors

• experience of reproductive specialists and embryologists;
• quality of embryology, andrology, genetics, and immunology laboratories;
• and in some cases microbiology, virology, pathology, and clinical laboratory services.

6. Could you explain these factors in more detail?

Age
Age is the most critical factor for women. Worldwide data show the highest IVF success rates in women under 35 years old. At that age, ovarian function is strongest and the likelihood of obtaining high-quality eggs and embryos is highest.

Often, one stimulation cycle can provide enough embryos for multiple pregnancies.

Male age also negatively affects reproductive outcomes, even when semen analysis results do not appear dramatically worse.

Cause of infertility
When the infertility cause is clearly identified, it is often treatable.

For example:

• blocked fallopian tubes prevent egg and sperm from meeting naturally;
• IVF bypasses this by transferring embryos directly into the uterus.

In male-factor infertility, ICSI helps fertilization by injecting sperm directly into the egg.

However, unexplained infertility cases are more challenging because all tests may appear normal while pregnancy still does not occur.

At Nadezhda Hospital, there is an increasing number of patients with combined infertility factors, repeated implantation failures, and recurrent miscarriages.

Egg, sperm, and embryo quality
Poor ovarian reserve, hormonal issues, embryos that stop developing, or absence of normal sperm all negatively impact outcomes.

Modern medicine still cannot truly improve gamete quality—it can only observe and assess it.

Various stimulation protocols are used for women, while special sperm-selection methods are applied for men.

Nutritional supplements may sometimes improve sperm quality, but because spermatogenesis takes approximately three months, supplements must be taken for at least that long before improvement can be assessed.

Uterus and endometrium
Various uterine abnormalities can interfere with implantation:

• thin or overly thick endometrium;
• endometriosis;
• adenomyosis;
• polyps;
• fibroids.

Precise diagnosis is therefore essential before IVF treatment.

Genetic causes
Approximately half of all embryos have abnormal chromosomal content. The percentage increases dramatically with age and can reach up to 95%.

Some patients also carry chromosomal translocations or monogenic mutations that predispose them to producing genetically abnormal embryos.

Immunological causes
Every embryo is biologically “foreign” to the mother’s immune system. Implantation depends on highly coordinated immune mechanisms within the uterus.

Laboratory quality

Laboratory quality directly affects IVF outcomes. Strict monitoring of all critical parameters is essential, including:

• incubator conditions;
• equipment calibration;
• servicing protocols;
• embryo culture conditions.

If embryo culture conditions are suboptimal, embryo development will suffer.

Experience of specialists and embryologists
Experience provides calmness, expertise, and the ability to react appropriately to unexpected situations.

Every patient, embryo transfer, and ICSI procedure is unique and presents different challenges.

A sufficient number of highly qualified embryologists is also essential to ensure smooth workflow and consistently high standards.

7. What do the data at “Nadezhda” show regarding IVF success rates?
According to Stefka Nikolova, the age distribution of ovarian punctures performed in 2024 at Nadezhda Hospital shows that most procedures involved women over 41 years old.

Because these patients often undergo natural-cycle IVF or mild stimulation protocols, they require multiple punctures to collect enough eggs and obtain balanced embryos for transfer.

Overall:

• 77% of punctures were performed in women over 36 years old.

For younger patients under 35, ovarian stimulation usually produces enough embryos for transfer after a single cycle.

The clinic now focuses increasingly on cumulative success rates by age group, tracking whether pregnancy is achieved from either fresh or frozen embryo transfers.