Hormonal imbalances The hormonal balance that controls both the woman's and the man's reproductive systems is extremely delicate, and any imbalance can prevent the couple conceiving.
Problems with the womb or abdomen Difficulties with the womb can make it impossible for some women to sustain a pregnancy.
A few women do not have a womb at all, usually because it has been removed in surgery.
Some have malformed wombs which cannot accommodate a growing baby, or problems with benign growths, or fibroids, in the wall of the womb, which distort it and can make it difficult for them to carry a pregnancy to full term.
Endometriosis This can be a painful condition in which tissue similar to that which normally grows in the lining of the womb is found in other parts of the woman's body, most typically on the outside of the womb or around the ovaries or fallopian tubes.
The causes and development of it are not clear, but the condition is usually graded mild to severe.
In its mild form, it does not interfere with fertility.
At its worst it can result in substantial adhesions around the ovaries, fallopian tubes and womb, or cysts within the ovaries, and will definitely interfere with fertility.
Cervical factors If the mucus in the woman's cervix is of poor quality, not conducive to sperm movement or hostile to the sperm, it can prevent them from reaching and fertilising the egg.
Sometimes the cervix will not retain a growing pregnancy, a condition called, in an appallingly derogatory way, an 'incompetent' cervix.