"Be grateful for what you've got" is the response that Tracey Jones, 37, gets whenever she speaks out about her heartache of not having a second child.
"Even close friends say it," she says. "It's as if they don't think I realise that I'm already blessed by having one child and there's this perception that I'm just being greedy."
In fact, Tracey refers to her four-year-old son as a "miracle." "I feel so incredibly lucky to have him. But I just don't feel our family is complete. I want him to have someone to grow up with and I want more noise in our home, the home we bought when he was little to accommodate our growing family. But I just can't seem to get pregnant again and the anguish can be overwhelming."
Tracey is one of many women suffering from Secondary Infertility. It's a term that many people have never heard of and even fewer understand. Even the British Fertility Society's press officer said, "What's that?"
Yet the condition, which is defined by doctors as the inability to conceive or carry to term a second or subsequent child, is on the rise. A US study revealed that in 1995 1.8 million women suffered from secondary infertility; in 2006, it was 3.3 million. Secondary Infertility now accounts for six out of 10 infertility cases.
Fertility expert Zita West, who has witnessed the increase first-hand at her London clinic, explains some of the reasons. "A lot of women are older when they have their first baby now, so age is an issue."
Lifestyle can also be significant. "Many women trying for a second baby are in the midst of what I call the 'first year fog.' Some have just gone back to work, then they do another day's work with the baby when the child care has finished, and they go to bed later because they want to achieve things in the evening. No wonder they're tired and exhausted. Many couples also tell me that their child is still in their room or their bed or they feel too tired to have sex as much."
Then there's the double-edged sword of stress. Women know the chances of conceiving are less if they're anxious, but it can seem impossible to relax when all they can think about is when their next child is going to come. "Many women obsess about the age gap, which piles the pressure even higher," adds West.
There can be medical causes too. "Something may have changed since you had your last baby. Perhaps you've developed anaemia or diabetes. Birth can put the thyroid out of kilter too. Maybe you're underweight or overweight or you're still breastfeeding. All these things can affect hormones and ovulation."
Some women discover medical problems that have always been there. "I was told that I'd probably had something significantly wrong all my life and that it was amazing I had conceived even once," says Louise Turner, 36, who was given fertility drugs that led to her having her second child this May.
The NHS recommends seeing your doctor if you haven't got pregnant after trying for a year (or six months if you're over 35). But Tracey says doctors didn't take her seriously.
"They implied I was impatient and paranoid and pretty much sent me away with a flea in my ear. I had to push my GP to fight my corner for invasive tests. Eventually, I was told that the likelihood is my eggs have aged prematurely, although they don't know for sure."
Like so many women who suffer from Secondary Infertility, the desire to have a child can be so powerful that a mere glimpse of a family with two or more children can make her well up.
"Yesterday, some friends had twins and my envy has almost been suffocating and that makes me feel really guilty."
Guilt, says HarleyStreet.com fertility specialist Anya Sizer, is the most common emotion among women suffering from Secondary Infertility.
"Women like Tracey berate themselves for not feeling more satisfied, particularly when the compassion fatigue among friends and family kicks in."
It's not as if there is anywhere obvious to go for support, says Susan Seenan, deputy chief executive at the Infertility Network UK. "Infertility forums inevitably include women who haven't had any children and these women just won't get why someone with a child feels so awful. But for many of those suffering from Secondary Infertility, it's as if their life is on hold and they constantly feel something is missing."
It doesn't help that children often start asking their parents why they haven't got a brother or sister, adds Yacoub Khalaf, Director of Guy's Hospital fertility and IVF centre. "It can make the parent feel helpless. Even other adults may tell you to get a move on with number two, assuming that because you have had one child, you can easily do it again."
But women shouldn't feel despondent, unless they've been told categorically they can't have another child, he says. "Many women do get pregnant again once they've had all the tests."
Don't put off the tests, he concludes – not just the woman but the man, whose sperm may, for example, no longer be healthy. Some women also swear by alternative therapy – hypnotherapy, acupuncture, reflexology and meditation. IVF is also an option.
Emotionally, cut yourself some slack, adds Seenan. "Yes, you have one child, but what's wrong with wanting two? And what's wrong with acknowledging that it's having a huge impact on your life?"
If things don't change, allow yourself some time to get used to it, she says.
"As terrible as it is that someone can't have their first child – and I do believe that is much worse – at least you don't know what you're missing," says Tracey. "With me, I've had this opportunity to enjoy every second, every smile, every milestone and it's unbearable to think of it never happening again."
Visit the Infertility Network UK or call free on 0800 008 7464