(CNN)"Nobody could tell us why we couldn't have children, but we've been trying at this point for about 12 years," said Elizabeth Prior, 49, of Buckinghamshire.
She and her husband tried in-vitro fertilization and adoption for several months but were advised against taking any of the children they met. They looked into fostering too, but Prior felt that nothing worked, "so we went down the surrogacy route."
They are among an increasing number of British couples who chose to pursue surrogacy, when one woman carries a child for another.
But UK law has many restrictive elements, which in addition to high cost and a shortage of surrogates in the UK makes many couples feel that going abroad is the only option.
Prior and her husband chose to carry out the process in the United States. "It was just so straightforward compared to how it could have been in the UK," she said of her experience.
The UK's legal limitations were highlighted in a recent study published in the journal Human Fertility that suggested the need for reforms. Of the 132 parents who completed surrogacy and were surveyed for the study, 92 underwent the process abroad, the researchers found.
"Surrogacy is legal, but it's restricted," said Natalie Gamble, who co-authored the study. Gamble runs a family law firm, NGA Law, and a not-for-profit surrogacy agency, Brilliant Beginnings. NGA Law claims to have helped over 1,000 families created through surrogacy.
The practice has been on the rise over the past couple of years, according to the study.
There are no statistics on surrogacy births, but the study suggests deriving an estimate from parental orders, when would-be parents apply to become the legal family of their child born through surrogacy.
These orders can be granted to couples only when one or both partners is the genetic parent of the child and one or both partners live in the UK; this rules out many family structures. The consent of the surrogate and her spouse are also required.
In 2007, 51 post-birth parental orders were issued. By 2016, that number rose to 316, according to Gamble.
This aspect of UK law leaves the possibility open that the surrogate could her mind about the process, as was the case in a 2015 lawsuit in England. The surrogate, who had entered an informal arrangement, refused to give the baby to its biological father and his partner. After a 15-month legal battle, the court ruled that it would be in the child's best interest to live with her biological father and his partner.
For Prior, the lack of legal security was her chief consideration in deciding to go abroad: the fact that there was a chance "that somebody would change their mind and keep the child that was destined for us."
According to law in the United States, where they sought their surrogate, "he was our child ... and that's just very, very reassuring," she said.
Gamble's study echoes this sentiment. Parents who traveled to the United States to use a surrogate were the least likely to have encountered difficulties returning to the UK, it says.
"Coming back from the US was seen as much more straightforward than coming back from countries such as India," said lead study author Vasanti Jadva, a senior research associate at the Centre for Family Research at the University of Cambridge.
A lack of surrogates in the UK also meant waiting for years, Prior said.
The sparse number of surrogates could be partly caused by one aspect of the UK surrogacy act, according to Gamble: The law doesn't allow advertising for surrogacy but instead allows nonprofit groups to facilitate surrogacy arrangements.
"This law was written in 1985, and there has never been a prosecution," she explained. The legislation didn't envisage the online world. On pages like Facebook, for example, there is a lot of advertising that breaches this law, but there is no "appetite for prosecution," according to Gamble. But, the UK's three surrogacy organizations are very careful to not break the law, so recruitment relies on word of mouth and media coverage, she said.
Kim Bradshaw, 36, of Portsmouth carried a child for a London-based couple who couldn't conceive because the wife had breast cancer and wasn't able to get pregnant while undergoing her breast cancer treatment despite being in remission for five years. Bradshaw has two children and wanted to be a surrogate "for the good."
"The lack of advertising makes it hard to find surrogates if they hadn't thought that it's something that they may consider," she wrote in an email. But if someone wants to seek out information, as was the case for her, they can.
Entering the process, Bradshaw wanted to be able to follow the family on social media, but they instead decided that she is "Auntie Kim" and will always be part of their circle. Bradshaw receives weekly photos and has had several visits with the family she helped create. The entire process "went to plan," and she is hoping to try again for a sibling of the child.
Surrogacy must also be altruistic, according to UK law, but it allows for "reasonable expenses" to be paid. There is no definition or upper limit of these payments, leaving a lot of uncertainty around the process.
Gamble's surrogacy agency has seen UK courts typically award between £8,000 (about $10,400) and £20,000 (about $26,000) as "reasonable expenses."
In the United States, however, Prior found that surrogacy agencies had a "bank of surrogates" from agencies, which allowed for a speedy process.
"All the i's were dotted and t's were crossed even before he was born," she said. The hospital in which her son was born had its own surrogacy coordinator. "That's how often they do it" in the United States.
Her family was able to return to the UK just 10 days after the birth of their son, now 2, and they are now expecting a second child from the same surrogate.