No, there is no scientifically proven incompatibility that would prevent a specific man and woman from conceiving together.
Some couples struggle to conceive for years, then separate and each have children with new partners. This does not mean they were incompatible. It more likely reflects differences in fertility between partners, better timing, or other biological and lifestyle factors that favour conception in the new relationship.
Why it can work with someone else
Fertility belongs to the couple, not to one person. If one of you is slightly less fertile, a very fertile partner can make up for it. If both of you are slightly less fertile, the small effects add up and it can take much longer. With a new partner, the balance changes.
Time matters too. A new relationship often comes with more frequent sex, and sometimes with a younger partner. Age is the strongest single factor in a woman’s fertility, and it affects men too, more gradually. Chance also plays a part: even for a healthy couple, each cycle is far from a sure thing, and most need several months.
Can your body reject your partner’s sperm?
Not in a way that would make you incompatible with one man in particular. Your vagina is naturally acidic, but semen neutralises it for a while, and around ovulation your cervical mucus becomes thin and stretchy to let sperm through. This happens with any partner.
Some people produce antibodies against sperm. Their real effect on fertility is debated, and testing for them is not part of a routine fertility check-up. The old “post-coital test”, which looked at sperm in the cervical mucus after sex, is no longer recommended because it does not predict pregnancy.
A true allergy to semen exists but is rare. It causes itching, burning or swelling after unprotected sex. It is uncomfortable, but it does not make pregnancy impossible, and it can be treated.
What about blood groups and genes?
Blood groups do not affect your ability to conceive. If you are Rh negative and your partner is Rh positive, this matters during pregnancy, not before. It is checked routinely, and an injection protects the baby.
Some research has looked at couples who share similar immune genes (HLA). The evidence is weak and inconsistent, and it is not tested in routine care.
What does matter is when both partners carry the same inherited condition, such as sickle cell disease, thalassaemia or cystic fibrosis. It does not stop you conceiving, but it raises the risk for your baby. A simple blood test can tell you whether you are carriers.
What usually explains difficulty conceiving
When a couple struggles, the cause is usually found in one or both partners: irregular or absent ovulation, blocked tubes, endometriosis, or sperm that are fewer or less mobile. A male factor is involved in about half of cases. Sometimes no cause is found, which doctors call unexplained infertility. That is a real diagnosis, not a sign that you and your partner are mismatched.
That is why both of you should be checked, not only you. For him, a semen analysis is simple and quick.
When to see a doctor
- No pregnancy after 12 months of regular sex without contraception, or after 6 months if you are 35 or older.
- Very irregular, absent or painful periods.
- A history of pelvic infection or surgery, or a known problem in either partner.
- Two or more miscarriages.
- You or your partner carry an inherited blood condition.
