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(CNN) -- It was the perfect summer day for a wedding. The bride was glowing. The groom was handsome. For nine hours, Tiffany Burke photographed the memorable moments.
"I took one break for 15 minutes, to eat really quickly," Burke recalled. "You don't want to miss the shots; that's what you are paid for."
Later that night, she pulled into the driveway of her Bellingham, Washington, home. She walked inside, put down her gear and ran to the bathroom.
"I just started puking and puking and puking," she said. "I was just grateful that I hadn't puked at the wedding or on the bride."
Burke is six months pregnant. She's already had two children. But this time, she's carrying a baby for someone else.
Her past pregnancies were relatively easy. She did have morning sickness with both her sons: Holland, 7, and Blake, 3. This time, it's more severe.
She is sick most days, all day. Anti-nausea medicine helps, but only to a point. Sleep is the only thing that makes her feel better. There are many days the 31-year-old stays in bed until 2 p.m.
"I have my off days, where I am crying a lot. In fact, I may start crying again right now," she said. "But I would totally do it again."
About two hours south of Bellingham, Natalie Lucich lives with her husband, James. James is Tiffany Burke's brother.
Natalie and James met on a blind date, fell in love and got married in 2008.
Lucich got pregnant within the first month of trying. She had an easy pregnancy, and she was thrilled when her son Hunter was born in 2010.
"He was the perfect mix of both of us," she recalls on her family's blog, which they've been using to share their journey. "Those huge eyes melted my heart."
After nurses whisked Hunter away to clean him up and check his vitals, a doctor began stitching Natalie up.
But an hour and three packs of thread later, she was still bleeding. Concerned, the doctor pushed on Lucich's uterus. It wouldn't contract. The pain was unbearable. She blacked out. She was losing blood quickly.
Lucich needed emergency surgery.
"I prayed two things before they began," she wrote. "I prayed that I would make it through safely for my husband and my son and that they would put me under because the pain was so horrible I didn't think I could handle any more."
James and her dad were by Lucich's side when she woke up after surgery. Unable to talk because she still had a breathing tube, she scribbled a note.
"Were you scared?" she asked. "Did they take my uterus?"
James told her the bad news. To save her life, doctors removed it. Lucich wrote another note. "It's OK. We can adopt."
It took a week for the news to set in. Lucich could have no more children naturally. Her dream had always been to have three. She was crushed.
A few days later, Burke headed over to the Luciches' house to take pictures of the new family. Lucich confided how conflicted she felt: She was grateful for Hunter but also mourning the loss of the children she would never have.
Lucich mentioned to Burke that she still had her eggs. She and James were considering using a gestational surrogate. An embryo, created in a Petri dish from Natalie's egg and James' sperm, would be implanted in the surrogate mother. None of the surrogate's DNA is involved.
"I was pissed!" Burke recalled. She was worried: What if the surrogate drank or smoked or did something to harm herself? She didn't want the Luciches to take that chance.
Suddenly, Burke blurted out, "I'll do it!" Embarrassed, she immediately covered her mouth and apologized.
Lucich looked at her. "I thought you didn't want any more children, you were done being pregnant."
"Yes," Burke responded, "we don't want any more natural children of our own ... but I have this perfectly good uterus!"
They hugged and cried and cracked jokes about Burke carrying her brother's baby.
But by the end of the visit, the sisters-in-law decided to talk with their husbands. What did they think about Burke helping Lucich become a mom again?
For weeks, both couples weighed the pros and the cons. Could Natalie and James afford it? It would cost them about $35,000.
Were there any health risks to Burke? After all, as they knew from Lucich's experience, pregnancy isn't without risk.
Finally, what about the emotional toll? "I remember thinking, what if I didn't want to give up the baby?" Burke said.
Four months later, all of them were ready to move forward. Burke would help James and Natalie expand their family.
Taking the next step
Experts say gestational surrogacy is growing slowly and steadily. According to the Society for Assisted Reproductive Technologies, 1,448 babies were reported conceived via gestational surrogacy in 2010 and carried to full term. There are no statistics on relationships between carriers and the intended parents.
Burke and Lucich contacted Seattle Reproductive Medicine, a medical practice that specializes in fertility issues.
Even though they were family, both couples had to hire attorneys and negotiate a contract that address a wide range of questions, from who would pay for Burke's doctors visits (the Luciches) to the legal process for James and Natalie to take custody of the child. In Washington state, it is illegal to pay for a surrogate's services.
Both couples also had to undergo a psychological evaluation to make sure they were prepared emotionally to go through the process. It took eight months from the time they decided to do it until the first embryos were implanted.
Finally, in January, Natalie's eggs were mixed with James' sperm. Then two embryos were implanted in Burke. That round didn't take. The next one did. Both couples were elated.
"I could not be more honored or more excited to be able to carry their child," Burke wrote in the family's blog. "I have also found new strength and support from my amazing husband, Sean. I know I could not do this without him. My love for him grows more and more each day."
Burke and James Lucich were both adopted at birth, and are not related by blood to each other. Burke wrote on her website that she would have served as a surrogate for a biological brother too.
Then in April, a surprise.
"When the nurse first started the scan, she knew right away there were two sacs. We were able to see the little heart beats of each one and we were all instantly crying," Burke wrote on the blog. An ultrasound showed that she was carrying two boys.
"They always say God has a greater plan than you could ever imagine ... well, He has far surpassed that. James and I couldn't be happier," Lucich added.
It was around the same time that some unexpected realities set in. Burke started getting sick and was exhausted all the time. That made it hard for her to work. It also meant the majority of the housework, cooking and child care ended up falling to Sean, an aspiring filmmaker.
Sean Burke is quick to point out that the Luciches and others help as they can but admits it's still difficult.
"It's been hard; I've been quite stressed out through this process," Sean Burke said. It's also been a strain on his budding career. "My career is just starting to kick off, and I've had to slow down a bit on that, and that's been tough."
Burke also feels guilty that she's missing this time with her boys. Her 7-year-old son, Holland, has especially been affected.
"I think sometimes that I want to help her a little bit. ... I just don't want her to be sick, really," he said.
Recently, things have been turning around. Burke has begun feeling better. She isn't getting sick as much.
"I feel like a human about two to four hours a day," she said. On a recent afternoon, Burke felt well enough to take her sons clothes shopping and, as a special treat, to Toys R Us to pick out a toy and then out to dinner.
The babies are due in December.
"I'm excited for the boys to be here, to see what they're like and for them to be with James and Natalie ... but in a selfish way, I am also looking forward to being done," she said.
Still, she has no regrets.
"These babies wouldn't exist if it weren't for this tough decision. There are a lot of new challenges I wasn't aware of, but I kind of think that's life, too. You kind of sign up for some things, and you hope you're strong enough to handle it, and hopefully you are. And hopefully, I am."
Dr. Sanjay Gupta and CNN medical producer Trish Henry contributed to this story.