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Lee Rubin Collins had her first baby in the mid 1990s, and the whole experience was such a joy, she says, she and her husband immediately said, "Let's keep doing this!" They got pregnant easily the first time around, so she was surprised when they tried and tried... and nothing happened. Ultimately, it took two years of fertility treatment, five cycles of in vitro fertilization, and one miscarriage before Collins was able to have another child, making her one of the millions of women in the United States who face secondary infertility -- a condition that is seemingly as misunderstood as it is common.

In honor of National Infertility Awareness Week, here's what not to say to women and couples who are struggling to conceive again, and what to say instead:

1. "You got pregnant once. It will happen again."

A woman who has already conceived one or more biological children but can't get or stay pregnant again is considered to have secondary infertility. And it's common. Estimates suggest that more than 3 million women in the United States have grappled with it. Yet the condition is often misunderstood by friends and family who assume that just because a woman had a baby once, she'll definitely be able to again.

Unfortunately, that's not true. "Just because a couple conceives their first child easily does not mean conceiving again will necessarily be as easy," said Dr. Katherine Melzer Ross, a reproductive endocrinologist with GENESIS Fertility and Reproductive Medicine in Brooklyn. The human body is complex, conception is complex and there are no guarantees. That's why doctors don't treat secondary infertility as though it's somehow less serious than primary infertility, she said.

2. "I guess that's the risk with waiting too long. "

Age can certainly be a factor in secondary infertility, said Ross, explaining that in one-third of cases, secondary infertility is due to issues with the father; one-third of the time it's the mom and one-third of the time it's simply unexplained. "With all factors being ideal (i.e., in a normal, fertile couple), the natural fecundity rate is about 20 percent per month," Ross said. "This number drops to between 5 and 10 percent at age 40."

But again, secondary infertility is incredibly complex. "To me, a myth is that [secondary infertility] only occurs because of age, because of a woman getting older after having one or two children," said Collins, who is now a member of the Board of Directors for RESOLVE -- the National Infertility Association. Collins had a fever during her first labor, which she later discovered was likely a sign of an infection in her fallopian tube. "It wasn't age," she said. "It was having my first baby that caused my infertility condition."

That is why it's equally misguided to tell a woman anything along the lines of, "If you just relax, it will happen again," or even anecdotes like, "I know someone who couldn't have another, and tried adoption, and then she got pregnant." If there were simple answers, reproductive medicine would not exist as a field, and millions of women and their partners would not be struggling every day to conceive.

3. "When are you going to get pregnant again?"

Thankfully, this isn't one people tend to say when they know a friend or family member is actively struggling to conceive, but it is something people feel surprisingly entitled to ask mothers, and it's really never OK. You never know who is dealing with fertility issues, or grieving a miscarriage -- and a woman's reproductive plans are not anybody else's business unless she first brings them up first.

In that vein, if someone seems like they don't want to share, take the hint. "Don't continue to ask prying questions about our family planning when I am giving you evasive answers," said Kate Hudson Walker, a volunteer with RESOLVE who says she faced both primary and secondary infertility. "Do you really want to know about my husband's crappy sperm, and my prematurely aging eggs?"

4. "At least you have one baby."

In Collins' experience, people who try and address secondary infertility often try to offer "perspective" on the situation, which often takes the form of the dreaded, "Well, at least you already have a healthy child." Of course mothers know that. They also physically ache for another baby. Those two feelings are not mutually exclusive, and the joy of being a parent does not necessarily diminish the pain of longing for another child.

"This is the most common [one, and] it doesn't acknowledge that I might desperately want another child, and that only having one child wasn't the family I hoped and dreamed I would have," said Walker. "It doesn't acknowledge that the vast majority of the world gets to make this decision."

Within the world of infertility itself, Collins added, there can sometimes be a rift between women with primary infertility and secondary infertility, which can be difficult to navigate. Difficult though it may be, it's not useful to compare them, she said. "When I was going through it, I would say to women [experiencing primary infertility,] 'Secondary infertility is so deeply painful to me,'" Collins said. "'When I think of having primary infertility, it takes my breath away.'"

What to say instead:

1. "There's nothing selfish about wanting another baby."

"Many women feel guilty or selfish if they have one healthy child and are going through fertility treatments for a second child," Ross said.

Rather than trying to offer a woman perspective on her situation, it's much more useful to try and offer understanding, Collins said. In that regard, nothing is more powerful than telling her that she is not selfish for wanting another baby. Dealing with infertility is painful enough. Shame shouldn't be a part of it.

2. "It's totally OK if you can't make it to XYZ."

One of the kindest things you can do for a woman going through secondary infertility is to remind her that she's not a bad person if it's too painful for her to do things like go to other people's baby showers, Collins said.

It can be a tricky balance. "Don't treat us like lepers," Hudson said. "We still want to be included in activities, but we may have to bow out if it is too emotionally charged for us." Just remember: Birthday parties can be tough. Even a simple trip to the playground, where there are lots of babies and siblings playing together, can be emotionally exhausting. Be sensitive to that, and try and engage your friend in activities that let them step away from the world of babies and children from time-to-time, Hudson encouraged.

3. "I'm so sorry, and I'm here when you want to talk."

It can be extremely challenging for women and their partners to pause and give themselves the time and space they need to acknowledge the pain of infertility while they're actively parenting another child. It can be very painful to watch as friends go on to get pregnant again.