Space sex - don't pretend you haven't thought about it. No one has investigated how human reproduction responds to changes in gravity, but new research on plant reproduction indicates that sex in space might not be as fun as you might imagine.

A Canadian study suggests that conception resulting from low-gravity reproduction could be dangerous to a developing fetus, possibly leading to growth retardation and health problems like brain disease.

The study, conducted by biologist Dr. Anja Geitmann of Montreal University and published in the journal PLOS One on March 13, found that plant cells growing in low gravity have damaged intracellular transport, which makes it difficult for them to communicate with each other and seriously hampers normal cell growth.

If human cells are affected in the same way, contraception might be necessary for astronauts attempting sex in space- for more than just the usual reasons.

Why Study Plant Sex in Space?

The focus of the study was the pollen tube-the reproductive cell that makes plant sex possible. Plant reproduction involves transferring male sperm cells, or pollen, to the female sexual organ in plants- the stigma. When a pollen grain lands on a stigma, it grows into a pollen tube that acts as a tunnel for sperm cells to reach the egg.

Since plant sperm cells are delivered to the egg by a "cylindrical tool," like in human reproduction, Geitmann figured that the growth of pollen tubes in microgravity might be analogous to human space fertilization. Pollen tubes are also the fastest-growing cells in the plant world, which made the effects of microgravity easy to observe in real time.

 "Researchers already knew humans, animals and plants have evolved in response to Earth's gravity and they are able to sense it," Geitmann told the Daily Mail.

"What we are still discovering is how the processes occurring within the cells of the human and plant bodies are affected by the more intense gravity, or hypergravity, that would be found on a large planet, or the microgravity that resembles the conditions on a space craft."

"Intracellular transport processes are particularly sensitive to disturbance, with dramatic consequences for cell functioning. How these processes are affected by a change in gravity is poorly understood.'

Geitmann's team used spinning centrifuges in the European Space Agency to simulate the effects of microgravity and hypergravity in space on plant sex. They put pollen tubes in the centrifuges and stained specific structures in the cells to show how gravity changes affected the transport of materials within the cells, then used real-time microscopy to watch the samples develop.

The results showed that the pollen tubes in simulated microgravity were significantly smaller, while the pollen tubes in hypergravity grew wider as gravity exerted more force. They also showed that altered gravity compromised the complex and highly coordinated networks that carry messages and materials within and between cells to promote normal growth.

"This allows us not only to understand general principles of the reproductive mechanism in plants but, more importantly, how the intracellular transport machinery in eukaryotic cells responds to altered gravity conditions," said co-author Dr. Youssef Chebli.

What Does That Mean for Human Sex In Space?

The disruption of normal plant sexual reproduction suggests that changes in gravity could have dire implications for human reproduction in space as well.

Intracellular transport is key to the healthy development of many human cells, especially neurons in the brain. If the growth of developing human cells is similar to the plant cell growth observed in this study, microgravity could cause major complications for the outcome of human sex in space.

Geitmann suggests that her team's findings "are significant with regards to human health, as a traffic jam on these highways that also exist in human cells can cause cancer and illnesses such as Alzheimer's.'

According to Livescience, astronaut studies by NASA showed cognitive performance declines in space, though the effects were inconsistent. The declines could have been caused by radiation or sleep deprivation, and no one has yet investigated the actual functioning of neurons in zero gravity.

Many neuronal diseases, like Huntington's, Parkinson's, and Alzheimer's disease, involve deficits in trafficking between cells, and Geitmann speculates that microgravity could have some effect on their development.

It seems that human reproduction in space has great risks- much more research needs to be done before conception via sex in space can be viable.

So ... When Will Space Sex Happen?

Though mixed-gender crews have blasted off into orbit since 1983, no one has ever reported any instances of sex in space. NASA has denied any instances of space sex during previous missions.

The Daily Mail points out that NASA doesn't forbid sexual activity during space voyages, though high professional standards, rigorous schedules, tight quarters, and constant monitoring from ground control are likely to dampen any tension among astronauts.

Still, it's only a matter of time before space flight planners will have to account for spacefarers getting down to business. Space missions are long enough as it is, and professional boundaries may not prevent sexual interactions for much longer.

Virgin Galactic expects to launch private trips into orbit by next year, and space tourists are likely to be less restrained than the company's name implies. A space voyage manned by a married couple, projected to happen as soon as 2013, would ostensibly not require a strict separation of work and pleasure.

In a mission that has little to do with science, retired porn star Coco Brown is training to be the first adult performer in space in 2014. It's unclear whether the private flight will yield any evidence of space sex- in public statements, Brown has been coy about the nature of her mission.

In the long run, self-sustaining colonies on the moon or Mars would depend on sex in space- a development that would require working around the kinks that microgravity might cause in human reproduction.

Listen to science writer Mary Roach talk about sex in space: