What Affects IVF Success? The Key Factors, Ranked | The Fertility Primer

Bottom line up front: The age of the eggs used is the factor that matters most in IVF. Close behind come how many eggs and embryos a cycle produces, and the cause and length of infertility. Lifestyle factors such as weight and smoking matter too, as does the quality of the clinic and lab. Paid add-ons sit at the bottom of the list: the UK regulator currently rates none of them as proven to raise birth rates.

When IVF works or does not work, people naturally look for the reason. Was it the protocol? The clinic? Something I ate? The honest answer is that IVF outcomes come from many factors, some fixed, some changeable, some purely chance, and they are not equally important. This guide ranks the main factors by how strongly the evidence links them to success, and separates the ones you can influence from the ones you cannot. For a detailed look at age-specific numbers, see our companion article on IVF success rates by age.

The factors at a glance

Factor	How strongly it affects outcomes	Can you change it?
Age of the eggs	The largest single effect	Only by using younger frozen eggs or donor eggs
Eggs retrieved and embryos available	Strong	Partly, through protocol choice or extra retrievals
Cause and duration of infertility	Moderate	Some causes can be treated first
Previous pregnancy	Small	No
Weight, smoking, alcohol	Recognised by the NHS as affecting chances	Yes
Clinic and laboratory quality	Real but hard to measure from outside	Yes, by choosing carefully
Add-on treatments	None currently rated as proven by the HFEA	Yes, by declining unproven extras

1. Egg age

Age dominates every large dataset. In a UK study of 113,873 women, age was the strongest single predictor of live birth after IVF (McLernon et al., BMJ, 2016). In current UK figures, the birth rate per embryo transferred using a patient's own eggs was 38% for ages 18 to 34 but 8% for ages 43 to 44 in 2024 (HFEA, 2026).

The key word is egg age, not the age of the person carrying the pregnancy. What generally counts is the age at which the eggs were collected, so eggs frozen years earlier, or eggs from a younger donor, bring a different starting point. That is why donor eggs change the outlook so much for older patients. For more on that route, see how egg donation works.

2. How many eggs and embryos a cycle produces

More eggs means more chances for at least one to become a healthy embryo, up to a point. In the McLernon study, the chance of live birth rose steeply with egg numbers up to around 13. Retrieving 13 eggs rather than 5 was linked to about 29% higher odds of live birth. Having spare embryos frozen in the first cycle was one of the strongest positive signals of all, with roughly 1.9 times the odds, because it means one retrieval can fund several transfers.

Ovarian reserve tests forecast egg numbers before you start. AMH and antral follicle count are the usual tests for this, and they predict quantity far better than they predict quality. The protocol and dose your clinic chooses aim to get a good number of eggs without risking over-response.

3. Cause and duration of infertility

Why you need IVF matters too, though less than people often expect. In the UK study:

Longer infertility was linked to slightly lower odds. Three years rather than six was associated with about 9% higher odds.
Tubal and male-factor infertility were linked to about a 10% lower chance at the start of treatment. Unexplained infertility and ovulation problems were linked to slightly higher chances.
Those diagnosis differences largely disappeared once the outcome of the first fresh transfer was known. In other words, how your first cycle actually goes tells you more than the label on your diagnosis.

Some diagnoses can be addressed before IVF to improve the odds. Your clinic should explain whether any apply to you.

4. Previous pregnancy

Never having been pregnant before was linked to slightly lower odds than having had a previous pregnancy, but the effect was small. A previous pregnancy is a mild positive signal, not a promise.

5. Lifestyle: weight, smoking and alcohol

The NHS lists body mass index and lifestyle factors such as smoking and drinking alcohol among the things that affect your chance of IVF success (NHS, reviewed 2025). In England, local NHS funding criteria may also require a healthy weight and not smoking. These are among the few factors fully in your control, and changes take time to make a difference, so start early. Our guide to preparing your body for IVF covers practical steps.

6. The clinic and the lab

Clinic quality is real but hard to see from outside. Stimulation decisions, trigger timing, lab conditions, embryo culture and freezing, and transfer technique all add up. UK registry figures show how much practice can shift outcomes: across the decade to 2024, births per embryo transferred climbed by about half, from one in five to three in ten, while the multiple birth rate fell from 14.4% to 3.2%, with 84% of transfers now using a single embryo (HFEA, 2026). Good clinics have improved success and safety together. Be wary of any clinic that chases headline rates by transferring several embryos at once.

Comparing clinics fairly is difficult, because each treats a different mix of patients. Our guide to reading clinic success-rate data explains how.

7. Add-ons: lowest on the list

Many clinics offer extras: embryo glue, endometrial scratching, time-lapse imaging, immune treatments, PGT-A and more. The UK regulator's traffic-light ratings are sobering. As of its 2026 review, no add-on is rated green, meaning proven by high-quality evidence to improve birth rates for most patients. Time-lapse imaging is rated black, meaning no effect. PGT-A, endometrial receptivity testing, platelet-rich plasma and some immune treatments are rated red, meaning potential safety concerns or evidence that they may reduce effectiveness (HFEA, 2026). An add-on may still be right in a specific situation, but it should never be the first thing you reach for.

Factors that are not fully understood

Some differences in outcomes are not explained by the factors above. Looking at UK patients aged 18 to 37 treated between 2022 and 2024, births per embryo transferred stood at 36% for White patients and 30% for Black and Asian patients (HFEA, 2026). The reasons are not fully understood, and gaps like this are a reminder that averages hide a great deal.

And then there is chance

Even with every factor in your favour, a good embryo transferred well may not implant, and a modest one sometimes does. IVF outcomes are probabilities, so a single failed cycle is not proof that something went wrong, and a success is not proof that everything was perfect.

Questions to bring to your clinic

Given my age and ovarian reserve, what is my estimated chance per transfer and per complete cycle?
Is there anything about my diagnosis that should be treated before IVF?
Which lifestyle changes would make the biggest difference in my case, and how long do I have?
How many eggs and embryos do you expect, and how will you aim for that safely?
Which add-ons do you recommend for me, and what evidence supports each one?

Frequently asked questions

What is the single biggest factor in IVF success?

The age of the eggs being used. It outweighs every other factor in large datasets, which is why frozen eggs from a younger age, or donor eggs, change the outlook so much.

Does my weight really affect IVF?

The NHS lists BMI among the factors that affect IVF success, and some funding bodies set weight criteria. Ask your clinic what target makes sense for you and how long to allow before treatment.

Will paying for add-ons improve my chances?

For most patients there is no high-quality evidence that they do. The HFEA currently rates no add-on green, and rates several red. Ask what evidence supports each one in your situation.

Is a clinic with higher success rates always better?

Not necessarily. Rates depend on the patients a clinic treats and how it reports them. A clinic that transfers several embryos may post higher pregnancy rates with more multiple-pregnancy risk.

If my first cycle failed, does that mean something is wrong?

Not by itself. Many people need more than one cycle, and chance plays a real role. What happened at each step of the cycle tells you more than the outcome alone.

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The Takeaway

Put your energy where the evidence is strongest: realistic age-based expectations, a protocol that produces a good number of eggs safely, treatable diagnoses addressed first, lifestyle changes started early, and a careful clinic. Leave unproven add-ons until last.