Earlier this year, the parents of a two-year-old girl consulted Dr Ann Tan, consultant obstetrician and gynaecologist at the Women and Fetal Centre, Mount Elizabeth Medical Centre, for their fertility problem.

The 34-year-old woman and her 37-year-old husband have been trying in vain for a second child since the birth of their daughter.

They had even gone for intrauterine insemination (IUI) four times while living overseas. IUI is the standard artificial insemination procedure.

Secondary infertility was Dr Tan's diagnosis for the couple.

A woman who has not conceived after a year of active unprotected sex is considered to be suffering from infertility. Secondary infertility is usually defined as the inability to conceive or carry a pregnancy to term after successfully and naturally conceiving one or more children.

For couples faced with secondary infertility, the reality is often hard to accept. The offspring they already have is a living, breathing testimony to their fertility. How could they possibly be infertile?

Dr Tan said: 'There is a misconception that once you're fertile, you'll always be fertile.'

Almost half of her patients who seek fertility treatment suffer from secondary infertility.

She said that the reasons for secondary infertility are not very different from those for primary infertility. The causes could be age, pelvic inflammatory disease, endometriosis, the development of uterine fibroids (benign tumours which grow from the muscle layers of the uterus) or ovarian cysts which impede successful conception.

Complications in a woman's earlier pregnancies can also lead to difficulty in conceiving again.

For example, a woman who has decreased blood pressure caused by a severe postpartum haemorrhage can suffer from hormonal problems that can lead to decreased fertility. It is not rare for operatic deliveries (a form of childbirth in which a surgical incision is made through a mother's abdomen and uterus to deliver one or more babies) to lead to pelvic adhesions.

These are bands of scar-like tissue that form between two surfaces inside the body. Inflammation from infection, surgery or trauma can cause tissues to bond to other tissues or organs. They can cause infertility by preventing the sperm from reaching the egg or interfering with fertilisation.

Dr Tan's patients who seek infertility treatment are usually couples in their late thirties and forties who had their first child at 35 years of age and above.

She said that the quality of the egg and sperm may decrease for couples who have not had another child in several years. She cited ejaculation and sperm abnormalities as common causes of secondary infertility.

After routine checks on the couple with the two-year-old girl, Dr Tan found the woman fit for conception but learnt that the husband had a low sperm count. They had in-vitro fertilisation (IVF) counselling.

The couple are expecting another child soon. The woman conceived after the first cycle of IVF treatment and is currently in her second trimester.

This article was first published in Mind Your Body, The Straits Times on Oct 30, 2008.