It might be because Horlock, unlike Brush, often uses her own eggs. She invites the couple to her house for an at-home insemination -- making a baby, but not the old-fashioned way. The father produces a sample and Horlock inseminates herself.
"It's quite invasive, you know, to have someone else's body fluids inside you," she said. "It's not nice, but it's necessary."
Horlock said her own two children are OK that the babies she has given away are biologically half hers.
"I think these days everybody has half-brothers and -sisters all over the place with remarriage," she said. "And I think as long as you are open and honest about it, they accept it and my children think of it as rather normal now."
But it has also created tension in the family.
"Being pregnant has caused a rift with my father," Horlock said. "The rest of my family are supportive. My father feels I'm giving away his grandchildren."
In 2004, Horlock got word from a furious father -- a client who said the baby she'd given him was not biologically his. A DNA test revealed that the father was Horlock's boyfriend.
Horlock said she offered the couple a return policy.
"They were very, very angry. I said, you know, 'Whatever you want to do, if you want me to take the baby back, I will do. If you want to keep him and adopt him, I'll support you,' you know, I can't do anything else," she said. "I can't turn the clock back, so I behaved as, I think, as responsibly as I could at the time."
Horlock said the couple asked for their money back and threatened to have her arrested for fraud. But Horlock argued that she and her partner had protected sex. Ultimately, the couple agreed to adopt the child.
Brush, by contrast, has no genetic link to her surrogate babies. She is always impregnated through in-vitro fertilization using someone else's egg.
In 2000, Brush was implanted with three embryos in hopes that one would take for a gay couple in Ireland. She was surprised with triplets.
But having triplets didn't mean that she got triple the pay. Brush said surrogates get a small stipend for each additional baby.
Because money wasn't her motivation, Brush thinks that she was addicted to being pregnant.
"I loved that people are excited and they'll pat your belly and they'll ask you questions about the pregnancy," she said. "And to feel the baby move and to actually have a life growing inside you."
But not everyone understands.
Brush's daughter Meg Fielder remembered that teachers and friends would comment on her mom's ongoing state of pregnancy.
There were even comments on the Internet, Fielder said, with posters saying things like "what do those poor kids and her husband have to put up with?"
Even so, Fielder said, "it was an amazing experience, and it's done nothing but enhance our lives."
Brush has developed strong loving relationships with most of her surrogate families.
Every year she is invited to now-8-year-old Cole's birthday party.
"I thought it was going to be just a business relationship and a business transaction, you know? I need a baby, you're going to have it for me, and that's it," said Cole's mother, Sarah Case.
Case said she came to realize that Brush wasn't running a business with her surrogacy.
"I knew the first time I talked with her that this was going to be a lifelong friend," Case said. "I knew it."