The most likely cause of secondary infertility is the changes to the structure of the uterus, cervix, and fallopian tubes that occurred when the womb was bathed in estrogen during the first pregnancy. Estrogen stimulates growth. It is necessary for the placenta to continue developing to keep up with the growing fetus. Huge amounts of estrogen flood the pregnant woman's body and they don't disappear the moment the child is born. Fat cells, in particular, interfere with the enzyme that reduces estrogen to its "recyclable" components. And mothers with newborns have much more to do than to worry about dieting! The same estrogen that made the pregnancy possible also stimulates growth in the lining of the uterus, but not necessarily all parts of it equally.
Endometriosis can cause thickening near the cervix so that sperm cannot enter the uterus to initiate a second pregnancy. Bits and pieces of uterine tissue broken off during childbirth can lodge at the ovaries, blocking the release of the egg during ovulation. When secondary infertility is due to unusual stimulation of uterine tissue by the high amounts of estrogen in pregnancy, the pregnancy drug Clomid, offered in the United Kingdom under the trade name Serophene, really isn't a good choice. If there is a mechanical barrier blocking the release of the egg or the reception of the sperm, then a drug that stimulates ovulation like Clomid won't solve the problem. When the secondary infertility is simply due to high levels of estrogen, however, Clomid can be extremely useful.
There is no ovulation during pregnancy. The extremely high levels of estrogen in the uterus signal the ovaries that a new egg is not needed. In some women, estrogen levels after pregnancy continue so high that the ovaries "think" the woman is still pregnant. Clomid changes hormonal balance so that the ovaries get the signal to ovulate in the middle of the woman's menstrual period and conception can occur. Whether Clomid is right for you is something you and your doctor need to determine, and a physical examination of the uterus will be required. Clomid cannot, of course, enable conception if the reproductive issues are with the male partner. But for a significant number of couples facing secondary infertility, Clomid is exactly the right treatment to allow for the conception of a second child.