You've Done Everything Right. Your Genes May Be the Missing Piece.

MTHFR is the gatekeeper of the methylation cycle, a biochemical pathway that regulates how your cells copy DNA, produce energy, and regulate inflammation. In reproduction specifically, MTHFR controls the methylation patterns that guide embryo development and neural tube formation. It also influences how homocysteine is metabolized, a compound that damages egg and sperm DNA at high levels.

Here’s the problem: the MTHFR C677T variant, carried by approximately 40% of the population, reduces this enzyme’s efficiency by 40% to 70%. That means your cells are processing folate and managing homocysteine at a fraction of the rate they should. You can take standard folic acid supplements and still be functionally depleted at the cellular level where embryo development happens.

For women, this manifests as poor egg quality, recurrent miscarriage, or chromosomally abnormal embryos. For men, it shows up as low sperm DNA methylation, which correlates with poor sperm motility and reduced fertilization rates. Both partners with MTHFR variants are at higher risk of passing genetic problems to offspring if conception does occur.

The FSH receptor sits on the surface of ovarian cells. When FSH (follicle-stimulating hormone) binds to it, eggs grow. Simple. Or should be. The FSHR N680S variant changes how sensitive this receptor is to FSH signaling. This matters most during fertility treatment, but it also affects natural ovulation.

The N680S S/S genotype, present in roughly 10% to 15% of women, creates a receptor that is less responsive to FSH stimulation. Women with this variant often have poor ovarian response during IVF: they grow fewer eggs, need higher drug doses, and may produce eggs of lower quality despite normal baseline FSH levels. Some women with this variant also have lower ovarian reserve or early signs of diminished reserve even in their twenties and thirties.

You feel this as irregular or light periods, difficulty conceiving despite tracking ovulation, or surprise poor response if you pursue IVF. Standard hormone panels may show normal FSH, which is why the problem gets missed. The receptor just doesn’t listen.

Estrogen does not work because estrogen is in the bloodstream. It works because it binds to estrogen receptors in the endometrium, the tissue that must welcome and nourish an embryo. ESR1 encodes the estrogen receptor. Variants in this gene change how strongly the receptor responds to estrogen, which directly affects endometrial receptivity.

The PvuII and XbaI variants, present in roughly 40% of the population, alter estrogen receptor function. Women with certain ESR1 variants have reduced endometrial receptivity, meaning the uterine lining is less capable of accepting an embryo for implantation, even if the embryo is genetically normal. This is particularly relevant for women doing IVF or those with recurrent implantation failure despite multiple high-quality embryos.

You experience this as recurrent miscarriage despite chromosomally normal embryos, failed IVF transfers despite good egg quality, or simply an inability to conceive despite ovulating and having healthy partner sperm. Ultrasounds look normal. Bloodwork looks normal. Your body simply isn’t prepared to implant.

CFTR encodes a protein that regulates fluid and electrolyte balance across cell membranes. Most people know CFTR because mutations cause cystic fibrosis. But even carriers, people with only one mutated copy, can experience male infertility. CFTR is essential for the development of the vas deferens, the tubes that carry sperm from the testes.

Carrier status for CFTR mutations, present in roughly 1 in 25 people of European ancestry, is usually asymptomatic. But in males, even carrier status can lead to congenital bilateral absence of the vas deferens (CBAVD), a condition where the tubes that transport sperm never developed. A man with CBAVD has normal sperm production but zero sperm in the ejaculate because there is no anatomical path for the sperm to travel.

You experience this as azoospermia (no sperm in semen) despite normal testosterone and normal testicular ultrasound. Your partner may be told he is infertile with no explanation. Genetic testing for CFTR reveals the cause. This is not testicular failure; it is a developmental problem with the reproductive plumbing.

DAZL (deleted in azoospermia-like) is a gene on the Y chromosome required for spermatogenesis, the process of turning testicular germ cells into mature sperm. Deletions in the AZFa, AZFb, or AZFc regions of the Y chromosome, where DAZL is located, directly block sperm production. These are not subtle variants. They are missing DNA.

Dazl deletions occur in roughly 1 in 2,000 to 3,000 males with infertility but in a much higher percentage of men with azoospermia (no sperm). A man with an AZFc deletion, the most common type, typically has severe oligospermia or complete azoospermia: his testes produce few or no mature sperm, no matter what. Testosterone levels are usually normal. The testes are present and appear structurally normal on ultrasound. But the machinery for sperm production is broken.

You experience this as azoospermia or severe oligospermia with no explanation, even after extensive hormone and imaging workup. Your urologist may tell you the problem is “unexplained male factor.” Genetic testing reveals the specific deletion and whether any sperm production is possible at all.

The androgen receptor (AR) is the lock that testosterone opens. It’s present in the testes, prostate, and throughout the body. How sensitive your AR is to testosterone determines how effectively your body responds to it. The AR gene contains a variable number of CAG repeats. More repeats mean lower receptor sensitivity to testosterone and reduced androgen signaling throughout the body.

CAG repeat length is common genetic variation, not a rare mutation. But men with longer CAG repeats (typically 23 or more) have lower androgen receptor sensitivity, which can impair spermatogenesis even when testosterone levels are normal. These men may have normal or near-normal testosterone in the bloodstream but their testicular cells aren’t listening effectively to the testosterone signal. Sperm production suffers.

You experience this as low sperm count despite normal testosterone levels, poor sperm motility, or difficulty conceiving despite baseline hormone testing looking acceptable. Your endocrinologist says your testosterone is fine. Genetically, your cells just don’t respond to it as efficiently as they should. Standard hormone therapy doesn’t fix the problem because the problem isn’t low testosterone; it’s low receptor responsiveness.