You fell pregnant so easily the first time, so how come it's not happening second time round? The sad news is that infertility can strike at any time.

The first time you fell pregnant you let out a secret sigh of relief: you weren't one of those unlucky women who had trouble conceiving, who had to endure the heartache of failed attempts and fertility treatments. After you settled into life as a parent, you and your partner began making plans for a little sibling to join the family.

But after months of trying, you realised there was a problem. How could this be? Could infertility really strike now, when everything worked before?

It most certainly can, and it's a condition the experts call 'secondary infertility', or sub-fertility. 'There are no guarantees about falling pregnant at any time,' says Dr Devora Lieberman, gynaecologist at Sydney IVF. 'It's interesting, because people think that because they've been fertile it will continue in the future.'

There are no accurate figures to show how often secondary infertility occurs, as it's often not reported if couples decide not to pursue fertility options. But doctors say it affects many couples.

Emotionally and psychologically, secondary infertility can be particularly gruelling. There may be a lack of sympathy given they already have children, or they may have trouble finding a doctor who'll take their concerns seriously. They may also feel guilty that they still want another child once they have another.

What's to blame?
So what could possibly cause things to suddenly stop working, when they worked fine the first time?

Age is one of the biggest factors. With many women until waiting until 30 or later before having their first baby, they may be close to 35 or older when they try for a second child. And it's at this time that egg quality starts to rapidly decrease.

Men's fertility too can decline with age. 'About 40 percent of infertility has to do with the male factor,' says Dr Lieberman. 'Male fertility doesn't decline to the same extent that female fertility does with age, but there can be a variability in sperm quality.'

Also consider if your lifestyle has changed since your first pregnancy. Along with ageing, you may have put on weight, or you and your partner may be drinking or smoking more, and all these factors can reduce fertility.

In some cases, the first pregnancy and birth may have caused a problem that affects the possibility of conceiving again. An infection can lead to the fallopian tubes not working properly. Or there may have been a pre-existing condition and you were just incredibly lucky to conceive the first time.

There are also other infertility causes that can develop, even after successfully conceiving one or more children. Endometriosis, where uterine tissues are located outside the uterus and make conception more difficult, can develop at any time.

Secondary infertility can also fall into the 'unexplained' category of infertility, where doctors can find no obvious cause. This covers about a third of all infertility cases, but the good news in these cases is that one- to two-thirds of couples with unexplained infertility will conceive within three years if they keep trying.

When to worry
How long should you keep trying to conceive before you consider there may be a problem and start seeking help? With a first attempt, doctors usually recommend couples try having regular, unprotected baby-making sex for at least a year before investigating problems.

But that may not always be the case if you've already had a baby. 'If you're 23 and haven't gotten pregnant in six months, you might actually think about some straightforward tests,' says Dr Lieberman. Likewise, if you're in your mid-30s, you may not want to wait a year and risk your egg quality deteriorating any further.

In terms of investigations, doctors will want to test the male partner's sperm to confirm the quality and quantity. For women, blood tests can confirm if a woman is ovulating, whether she has polycystic ovarian syndrome, or if there is an underlying pituitary or ovulatory disorder.

Treatment options
The treatment options for secondary infertility are exactly the same as those for regular infertility.

In some cases it may be a matter of treating a condition like endometriosis, a blocked fallopian tube or fibroids with minor surgery.

If the trouble relates to your ovulation patterns, doctors may recommend charting your temperature and testing your LH levels with a store-bought kit to pinpoint your exact time of ovulation.

In some cases, women may be injected with hormones, or fertility drugs, to stimulate egg growth and ovulation. In others, your only option may be in-vitro fertilisation (IVF).