Surprise pregnancy? Anatomy expert reveals sex acts that aren't as conception-proof as people might believe...

Safe sex is vital for young couples who don't want children, but some methods might not be as pregnancy-proof as you think. 

In fact, women have conceived children in some surprising ways that seem to defy the laws of biology.

One factor that unites them all is the resilience of the human reproductive system and its ability to perform against the odds.

Professor Adam Taylor, of Lancaster University, has detailed how women can get pregnant in four extremely rare circumstances — from anal sex, oral sex, a 'splash' insemination without any penetration whatsoever, and while they are already pregnant.

Getting pregnant from anal sex is extremely rare but it can happen in people with a reproductive abnormality called a cloacal malformation.

Anal sex might seem like a safe choice for couples trying to avoid pregnancy, but a rare condition can allow sperm to reach the ovum

The only really safe way to avoid pregnancy is to use contraception, say experts

This is where the divide between the dividing tissue between the vagina and anus fails to form properly. 

When this happens, it may allow sperm to swim through any opening in the dividing wall towards the egg to fertilise it.

Professor Taylor said sperm are naturally attracted towards the ovum by 'sniffing out' trace amounts of chemicals it produces, and they will take any route necessary to reach their goal.

'They're able to detect trace amounts of chemicals that the ovum produces,' he told The Conversation.

'As the sperm swim towards the ovum, the amount of these "chemoattractants" they detect increases, signalling them to continue travelling in the right direction."

Cloacal malformation affects one in every 50,000 girls and require corrective surgery.

There is a high probability of complications with this surgery such as incontinence and difficulty getting pregnant.

Medical records contain details of a pregnancy caused by a bizarre set of circumstances that started with oral sex in 1988.

A 15-year-old girl living in the southern African nation of Lesotho went to her doctor with abdominal pain and was found to be heavily pregnant.

Oral sex should be a safe bet to avoid pregnancy, but the determination of sperm to reach their goal should not be underestimated

A girl who was born without a vagina ended up pregnant in 1988 after a traumatic experience that started with oral sex (stock picture)

But incredibly, the girl had been born without a vagina, a condition called Mullerian agenesis — leading everyone to wonder how this had happened.

It transpired that, nine months earlier, she had been engaged in fellatio with her lover and was caught in the act by her jealous ex-boyfriend, who promptly stabbed her in the stomach.

She went to hospital and was patched up, but not before her lover's stray sperm had made its way into her system.

Sperm are known to be able to survive in the peritoneal cavity, the space between the abdominal organs and body wall.

In the girl's case, her lover's hardy swimmers managed to make it to her ovum and fertilise it, possibly via her gastrointestinal tract.

Doctors delivered a healthy baby boy via Caesarean, according to a report in the British Journal of Obstetrics and Gynaecology.

There are other ways for women to become pregnant, even if they haven't had sexual intercourse.

'If semen splashes against the external genitalia, sperm may find their way into the vagina and swim towards the ovaries,' explained Professor Taylor.

Women can become pregnant during sexual activity, even if they have not had full sexual intercourse 

Healthy sperm have great resilience and can survive for half an hour outside of the human body 

So-called 'splash pregnancies' are highly unlikely because sperm can't survive for more than half an hour outside the body.

Healthy sperm swim at up to 5mm a minute and can survive for up to five days in women's genitalia.

Hundreds of millions of sperm are released into the vagina during sex in ideal conditions, but only 200 to 300 will make it to the egg — demonstrating how splash pregnancies are so rare.

They cannot occur from sperm in bathwater or hot-tubs, because the water disperses the sperm and dilutes the seminal fluid that protects them.

Chemicals such as chlorine in the water also rapidly kill sperm.

In very rare cases, it may be possible for a woman to become pregnant a second time while she is already carrying a baby.

Called a superfetation, this phenomenon throws out all the 'safety measures' that usually prevent a second pregnancy.

Hormonal changes during pregnancy usually prevent a second pregnancy from occurring 

In very rare cases, called a superfetation, a second egg can become fertilised while a woman is already carrying a baby

During pregnancy, a woman's hormones change to prevent an embryo from implanting in the uterine lining.

Even if a second embryo were to form somehow, it's unlikely that it would be able to begin growing in the uterus.

This is true even for women who are born with two uteri, as these mechanisms work hard to prevent a second pregnancy happening.

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Hormones prevent ovulation and produce a thick mucus plug that covers the cervix to prevent sperm travelling into the uterus towards the ovary.

But very occasionally, usually in women undergoing IVF treatment, these safety measures can be overridden.

The two pregnancies often happen very close to each other, usually within two to four weeks.

While the babies will not be twins, it means they can usually be delivered together at the same time.

Most of these pregnancies progress normally without complications.

Professor Taylor said: 'These examples are of course extremely rare – so you probably don't need to worry too much. But if you aren't looking to get pregnant anytime soon, be sure to use contraception.'