You've Tried Everything and Still Can't Conceive. Here's the Biological Reason.

MTHFR encodes an enzyme that converts folate into its active form, methylfolate. This active form is required for hundreds of reactions in your body, including the methylation of DNA itself. In your eggs and your partner’s sperm, proper methylation is essential for healthy development. It’s how your cells know which genes to turn on and off during embryo formation.

The C677T variant, carried by roughly 40% of people with European ancestry, reduces MTHFR enzyme activity by 40 to 70%. This doesn’t mean you can’t conceive. It means your body is converting folate into usable methylfolate at a fraction of the normal rate. Your eggs and sperm are being built with suboptimal methylation, which can compromise embryo development and increase miscarriage risk.

You might have normal folate levels in your bloodwork and still be functionally folate-depleted at the cellular level. You may experience repeated early miscarriages, poor egg quality despite a good count, or slow fertilization. Your partner might have low sperm motility or poor DNA integrity. Neither of you feels obviously sick. But conception is a methylation-intensive process, and you’re running it on a broken engine.

FSHR encodes the follicle-stimulating hormone receptor, which sits on your ovarian cells and listens for the FSH signal that tells your eggs to grow. If your receptor is sensitive, you respond robustly to FSH and produce many mature eggs. If your receptor is less sensitive, you need more FSH to get the same response. This is not a problem in a natural cycle. It becomes critical in IVF.

The N680S variant, particularly the S/S genotype, occurs in roughly 10 to 15% of women. Women with the S/S variant tend to have a poor ovarian response to standard FSH doses in IVF stimulation protocols. They retrieve fewer eggs despite normal egg counts on ultrasound. They may need higher FSH doses, longer stimulation cycles, or different medication protocols to achieve adequate response.

If this is you, you may have been told you have “poor ovarian response” or “low ovarian reserve” despite having a normal antral follicle count. You feel like your ovaries aren’t cooperating with the medication. Previous IVF cycles might have been canceled due to inadequate response, or you retrieved very few eggs despite high drug doses.

ESR1 encodes the estrogen receptor, which lines your endometrium and tells your uterus when it’s ready to receive an embryo. Estrogen binds to this receptor and signals: “Now is the time.” Without proper estrogen receptor function, your endometrium may look thick on ultrasound but feel unwelcoming to an embryo at the cellular level. This is called poor endometrial receptivity, and it’s one of the most overlooked causes of implantation failure.

ESR1 variants, including the PvuII and XbaI polymorphisms, are carried by roughly 40% of the population. Certain variants reduce estrogen receptor sensitivity in your endometrium, making implantation less likely even if your embryo is perfect. You may have transferred beautiful embryos and gotten negative tests, or gotten pregnant only to miscarry. Your endometrial thickness on ultrasound looked fine. The problem was receptivity, not anatomy.

You’ve likely heard the phrase “unexplained implantation failure.” This is often what’s happening. Your embryo lands in a uterus that’s literally not listening to estrogen properly. All the embryo care in the world can’t fix a uterus that isn’t ready.

CFTR encodes a chloride channel protein that’s famous for its role in cystic fibrosis. But it also controls the development of the vas deferens, the tube that carries sperm from the testicles to the urethra. In males, even carrier variants of CFTR can cause a condition called congenital bilateral absence of the vas deferens (CBAVD), where the tubes that transport sperm simply don’t form.

CFTR carrier variants occur in roughly 1 in 25 people of European ancestry. Male carriers of CFTR mutations may have obstructive azoospermia: sperm that is produced normally but cannot be ejaculated because there is no clear path out. His semen analysis shows zero sperm. You’re told he’s infertile. But the sperm are there. They’re just trapped.

If your partner has been diagnosed with azoospermia and his hormone levels are normal, if he has normal testicular volume and normal semen characteristics except for the complete absence of sperm, CFTR should be on your radar. This changes everything about how you approach treatment. It’s not about sperm quality. It’s about plumbing.

DAZL and the broader AZF (azoospermia factor) genes on the Y chromosome encode proteins essential for spermatogenesis: the process of turning stem cells into mature sperm. These genes are like the instruction manual for sperm development. Without them, the process stalls.

Deletions in the AZFa, AZFb, or AZFc regions occur in roughly 1 in 2,000 to 3,000 males with infertility. Males with AZF deletions produce either no sperm (azoospermia) or very few sperm (severe oligospermia), despite normal hormone levels and normal testicular anatomy on ultrasound. The testicles look fine. The hormones look fine. But the factory isn’t producing.

If your partner’s semen analysis shows azoospermia or severe oligospermia (fewer than 5 million sperm per milliliter), and his hormone levels and testicular exams are otherwise normal, an AZF deletion is a real possibility. This is the genetic equivalent of a broken assembly line.

The AR gene encodes the androgen receptor, which sits on cells throughout your partner’s body and listens for testosterone. In the testicles, a sensitive androgen receptor is essential for spermatogenesis: the process of making sperm. Testosterone binds to the receptor and signals “make more sperm.” If the receptor is less sensitive, the same testosterone level produces fewer sperm.

AR contains a variable number of CAG repeats. The number of repeats affects how sensitive the receptor is. More repeats (typically 20 or more) mean a less sensitive receptor. This is common and not necessarily abnormal, but it shifts your partner’s sperm production lower on the normal spectrum. Men with longer CAG repeats produce fewer sperm despite normal testosterone levels because their cells aren’t responding to testosterone as robustly.

Your partner’s testosterone comes back normal, but his sperm count is low to low-normal. You’ve tried everything to boost his testosterone. But the problem isn’t testosterone production. It’s testosterone sensitivity. His cells aren’t listening properly.