Trying to get pregnant can be an exciting time, but it can also be disappointing and frustrating if things don't go to plan.
And while it's probably unreasonable to expect to get knocked up your very first go (though, believe us, that can happen), it's a different scenario when you've been trying for months on end with no result.
For many couples, the next logical step is to speak to a professional to double check everything is okay. This could be as easy as getting advice as to exactly the right time to try (like did you know a human egg is only viable for 12 to 24 hours?) or it could involve some further testing, such as semen analysis.
But what if you both get a clean bill of health, you're doing everything right and still no luck?
What does it mean?
"The first thing to say is unexplained infertility traditionally has been defined as a couple who have been trying for year and all conventional tests have come back as normal," Dr David Wilkinson from City Fertility Centre Melbourne told HuffPost Australia.
"But you have to make sure it really is unexplained. Sometimes the history can be sexual issues or timing issues, that kind of thing. Female age is another influence, as we know natural fertility rates beyond 35 do decline.
Model Megan Gale, stunning and expecting at age 42.
"So assuming the female in the couple is under, say, 38, and they are having regular sex at the right time, and they've had the sperm test, checked the ovulation, checked the thyroid, had the full history; assuming that's all done and everything has come back as normal, then that's what we would call unexplained fertility."
This can often be a frustrating diagnosis for couples, who naturally want to know, 'If everything is coming back okay, then why aren't we falling pregnant?'
Fertility tests and their limitations
It's important to note that unexplained infertility doesn't necessarily mean nothing's wrong, but that nothing is showing up in your tests to explain what may be the cause. In other words, the doctors simply don't know.
"But the tests are very basic, frankly, outside of a research setting. For instance, an example is the sperm test. We can count the sperm and observe their movement and how the sperm look, but all of that doesn't tell you if they can get into eggs or not. Some men's sperm look fine but they can't get into the egg.
"On the other hand, some men's sperm don't look that great but are actually fine. That's an important point. Very often a sperm analysis will show a minor abnormality, especially in terms of how the sperm look, but if that's the case you mustn't fall into a heap. Often it does not mean anything in terms of ability to conceive. Really. And, more than 50 percent of the time, when a sperm test is even moderately abnormal, it will be normal next time.
"We also do a test to check for ovulation. However we know that over 90 percent of women who are having regular cycles will be ovulating. Sometimes it's hard to work out when in the cycle they are ovulating, so that can be one barrier to overcome, but most of them are [ovulating].
"And often the ovulation test will be performed either too early or too late in the cycle to show ovulation. Women's cycles are variable: some women will have shortish cycles, like less than 28 days, and some will have longer cycles of more than 35 days. All with good quality ovulation.
"We often do a test to make sure the eggs are reasonable quality, so that's another basic test, called FSH (follicle stimulating hormone).
In over 50 percent of cases for couples who haven't had success, all those tests will be completely normal.Dr Kate Stern
"And there is another test, AMH (Anti-Mullerian Hormone), which people call it the egg timer test. It's a rough indicator of your egg reserve or how many eggs you have left. AMH is hard to interpret and really often the results are misinterpreted. My goodness, sometimes it can be a disaster. If the test result is low, some women get worried that they don't have any good eggs left and that is not true! It's a very vague indicator of ovarian reserve, but really, yours can be really low and you still can get pregnant.
"We can do a special ultrasound make sure the fallopian tubes are open. Those are the basic tests we do.
"And in over 50 percent of cases for couples who haven't had success, all those tests will be completely normal."
Yep. You read that right. Unexplained infertility is more common than you think.
"In my experience, the incidence of unexplained infertility is a lot higher than what we were taught in training," Wilkinson said. "I believe we were told it was something like five to ten percent of couples, whereas in my practice I would say it's more like 20 percent."
What might be wrong?
As previously outlined, unexplained infertility could point to the fact there is an actual issue with your ability to conceive, but the tests currently available aren't able to pinpoint the cause.
So what do the experts think could be going wrong?
"Well, there are a couple of popular ideas or old wives tales that have been debunked," Stern said. "Like the idea that the vaginal mucus isn't compatible [for the sperm's survival] -- that's gone.
"It's actually much more subtle than that. It could be a issue in the eggs. Sometimes it's the ability of the sperm to get into the egg, and that can be an egg issue or a sperm issue.
"There could be some inflammation inside the uterus. If you've had abnormal pap smears and had treatment which might scar the cervical opening, occasionally that can pose an issue, but really the general rule is, if the period can come out, the sperm can get in.
"As I said before, it's not that it's nothing, it's just that there are things that can't be picked up."
Wilkinson agrees, stating he believes we will find out a lot more about potential egg and sperm issues in years to come.
"My thoughts are, down the track as we find out more, is that there will be egg problems we can't [currently] diagnose and sperm problems we can't currently diagnose.
"My gut feeling is, as we learn more, quite a lot of it will come down to problems with eggs or sperm we can't identify as yet. Egg quality, in my opinion, is probably quite a big player."
Options going forward
First of all, don't despair. As we've already established, unexplained infertility is more common than you might think, and still many couples go on to have healthy pregnancies.
"Unexplained infertility is very common. I suppose there are a whole series of things that can be contributing and they tend to be different at different ages, so you really would have to look at the individual in terms of deciding what was right for them," Dr Sonya Jessup of Sydney's Demeter Fertility told HuffPost Australia.
Chances are, if you've been diagnosed with unexplained infertility, you're more than familiar with the idea of monitoring your cycle. But Jessup said you'd be surprised how easy it is to just miss the window.
"The egg is egg only viable for 12 hours, so if you're getting it almost right but not exactly right, that in itself can cause a delay in fertility.
"Often when people are young, I'll tell them to try ovulation monitoring, and tell them the exact time to have sex. Often that's all that needed."
Laparoscopy / Fallopian flushing
"The number of women I see with unexplained fertility who, when we do some tests, might have some endometriosis, is quite high," Jessup said. "And if there is any suggestion of endometriosis I will either do laparoscopy [a surgery to check your tubes and condition of ovaries and uterus] or flush some Lipiodol through their Fallopian tubes.
"These women, the vast majority of those for me are pregnant within a couple of months."
Indeed there is research to suggest tubal flushing can improve fertility and reduce the need for IVF, the idea being it helps remove any debris or blockages. Jessup goes even further, stating she believes there is a quality in the Lipiodol which assists the process.
"In my opinion in might have some effect on the lining of the womb, helping calm down some of the immune problems people have.
"We are unsure what the effect is at this point, but with the number of people getting pregnant, there does seem to be one."
Artificial insemination / IUI
According to IVF Australia, "Artificial insemination or intrauterine insemination (IUI) involves inserting the male partner's (or donor's) prepared semen through the neck of the womb (cervix) and into the uterus, close to the time of ovulation."
To heighten their chances of conceiving, often couples will choose to use medication to help stimulate the ovaries, as well as a 'trigger injection' to bring on ovulation.
The male's semen is also washed and prepared to ensure the final product (i.e. the bit that will be inserted into the uterus) is at its most concentrated.
While Stern is positive about the procedure, stating even the injections can help provide the "gentle boost" some couples need to conceive, Wilkinson is less enthusiastic.
"In term of artificial insemination, if you really look at the pregnancy rates -- assuming there's no sexual problem -- the rates aren't that much better than trying naturally.
"At the end of the day you want to do something that increases your chances.
"Don't get me wrong, artificial insemination is a fantastic treatment if we are looking at donor sperm [for a single woman] or a lesbian couple but the pregnancy rates aren't that great.
"Often comes down to very personal decision from the couple or the woman, including how old she is and how long she has been trying. I would discuss options with them and probably advise to keep trying naturally a bit longer, and if that doesn't work, go to IVF."
IVF is an artificial procedure by which an egg and sperm are conjoined externally, in a specialised laboratory. It requires the women to inject herself daily with hormones stimulate fresh egg production, and, if that goes well, her eggs are then collected, fertilised, and returned to the uterus where it will hopefully take hold.
While for many the expensive and invasive process of IVF is considered a last resort, all of the experts interviewed for this story have stressed repeatedly that age is an important factor that shouldn't be overlooked.
"I see a lot of couples where the female partner is already 40 when they start, and to be honest, the chances aren't that good," Wilkinson told HuffPost Australia. "In that situation my advice would be to go straight to IVF."
To find out how to find the best IVF clinic for you, head here.