this is "nightline." >> tonight they shed hundreds of pounds right before our eyes. on the tv show the biggest loser. many of those people gained most of that weight back. the surprising new study that reports a troubling finding about weight loss. why your body not your will power may be to blame. plus when janet jackson leaked a video online. was it a hint. she may be getting no sleep with soon she's pregnant at the age of 49. can you do it at age 50. so many of us hate getting stuck next to fussy fliers.

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we start with news from the world of weight loss. scientists followed contestants from "the biggest loser." they found when the cameras went

off, the weight often came back on. there are hard and surprising findings about our bodies. here's lindsey davis. >> reporter: it was a record-breaking moment. after shedding an astonishing 239 pounds, danny cay hill became the contestant to lose the most weight ever on the biggest loser kwth. he went home with a quarter million dollars literally half the man he used to be. >> your starting weight is 430 pounds. but it was what came after the afterphotos that proved the biggest shock to danny and many of the other contestants. >> i didn't want anybody to know i was on the biggest loser. i didn't look like that anymore. >> reporter: little did they know their metabolism dropped. what would turn out to be a

setback for the majority of contestants would prove to be a breakthrough to understanding why so many dieters struggle to keep off the weight they lose. in a new study that has everybody talking, scientists followed 14 of the 16 contestants from season 8 to the biggest loser kwth. monitoring them for six years after the show. all but one gained some of the weight back. four of them reported being heavier now than at the beginning of the show 123467? >> when you attempt to fight your body weight, your biology will fight back. >> reporter: this scientist says while they had a normal metabolism at the start. years after their metabolism slowed down. a mechanism he says the body uses to return to the original weight. >> your body's metabolic rate will slow down. the number of calories you need is going to decrease.

that makes it difficult for you to keep the weight off. >> are you ready? >> yeah. >> reporter: huge implications at a time when we're inundated with weight loss shows and celebrities going public with their struggles. >> i have eaten bread every single day. >> reporter: struggles that six years later danny continues to face. >> i've had people look at me and tell me i ought to be ashamed of myself. i ought to walk around the block a few times. >> reporter: as the winner he felt he also had the most to lose. 430 pounds before the show, 191 pounds right after. and since then he's gained back more than $100 pounds. >> was this sustainable the weight loss that you had? >> it's not at all. when i went to the ranch i was there for 90 days. i quit my job. that's why i won the show and that's why i did what i did. i made it my job. but we're finding out that in order for us to keep our calorie

burn at the same rate of a normal person, that we're having to work out two hours a day. >> reporter: he says in order to maintain his current 295 pounds, he has to eat 800 calories less than a typical man his size. >> i started eating like a normal man would to be even. and i wasn't staying even, and that was what was dishartening, and i heard this from all the biggest loser contestants. >> it's difficult to keep weight off because your body doesn't want to do it and your desire to eat increases. it's the perfect storm for weight regain. >> reporter: it might explain why there were plummeting levels of leptin among contestants. >> i kind of failed, here i went on national tv and became this person for people to look up to and now i've gained this weight. >> reporter: she was also part of the study. she was 248 pounds before the

show. down to 173 at the end. and now back to 204 pounds. she says the results of study have helped her understand why it's taken so much effort to maintain her ideal weight. >> i have no idea what it's going to be like in 15 years as my metabolism continues to slow. i guess that's what i'm afraid of. >> reporter: a fear that's led her to question her decision to join the show in the first place. >> had i known what i know today, i probably would have not done the show. >> reporter: but the creator of the biggest loser kwth defends his show. >> when people leave their show they're better off than being 400 pounds. i think the study could have done things a little differently. i would like them to take 14 people who kept the weight off. >> reporter: he said the studies may have unintended negative results. >> any time you tell somebody it's not them, it's something else, it gives them kind of a

way to excuse the fact that they really want to eat that. >> reporter: he says for him and many others it's helped relieve some of the crushing shame that came from the weight gain. >> i've heard from obese people all over facebook and twitter, everybody is messaging me going i knew this was going on. this makes me feel better about where i am. not an excuse, but at least some of the shame is lifted and now we say where do we go from here. >> reporter: one of danny's closest friends on the show, 26-year-old amanda. >> it's great the world knows that biggest loser contestants, we aren't lazy and didn't waste an opportunity and we're not just gaining weight back. >> reporter: over the course of the season she lost 87 pounds. since the show wrapped seven years ago she's now gained back 12 of those pounds. giving hope to many that weight loss can sometimes be maintained. and joining a list of

celebrities who have gone public with weight loss that appears to be permanent, but she says it's a difficult daily struggle. >> to maintain my weight, i have to eat less than 1400 calories a day. it's almost nothing. i'm not full off that much. and i find myself in a constant, well, maybe i shouldn't eat right now, and it is almost like an eating disorder in way. it's like i have to restrict everything i do. >> reporter: like danny, she breathed a sigh of relief when the most recent study was released. finally an explanation for her struggle. she's concerned about the long term ramifications. no. >> now i know i'm burning 600 calories less per day than i was six years ago. what does the future hold for me? it's like am i going to gain the weight back? is my destiny going to be an over weight woman because my

body is fighting me. >> reporter: she says one of the most important motivational tools is of all things instagram. >> i've made sure that i go on social media a lot. i have over 83,000 followers on instagram. i want to be as real as i can with people. i found that going that keeps me accountable. >> reporter: tonight add to that one and a half million "nightline" viewers. but no pressure. i'm lindsey davis in new york. >> up next, it turns out there may be a deeper meaning behind janet jackson's new song dam baby. if she's pregnant at 49, how did that happen and can it work for you?

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the tv show "good times". then she made her name as miss jackson if you're nasty, and now we may be watching her at about 50 to become a mom. >> her new single says it all. janet jackson may be expecting her first child with her husband, a businessman. according to a report by entertainment tonight and other outlets. just shy of her 50th birthday, the superstar postponed her unbreakable world tour last month. surprising fans with this twitter post. >> and there has been a sudden change. i thought it was important that you be the first to know.

my husband and i are planning our family. so i'm going to have to delay the tour. please, if you can try and understand that it's important that i do this now, i have to rest up. >> reporter: the tour, part of jackson's long awaited comeback releasing new music for the first time since 2010. and winning the ultimate icon award at last year's bet awards. >> my heart is so full. >> reporter: how are you feeling right now? >> i'm very well, thank you. >> when it comes to family, janet has taken a hands on role in the lives of her brother, michael's children. >> i want to say i love him so much. >> reporter: publicly comforting one of them at a memorial. as for having her own children, the star saying after her brother's death it would be very sweet if i did have children. the icing on the cake.

♪ >> reporter: but with her nearly 50 years old, getting pregnant is challenging. >> a spontaneous natural pregnancy in a 50-year-old woman is almost unheard of. it's not that it never happens, but it's incredibly rare. it's either the use of donor egg or the woman's own egg that she had frozen years before. >> reporter: janet wouldn't be the first celebrity to start a family later in life. halle berry showing off her baby bump at age 46. and always be my baby singer mariah carey opened up with her struggles about infertility. >> did you have fertility treatments to try to get pregnant? >> the main thing i did was

tough, and that was to go on progesterone every month. and then when i was pregnant i had to stay with the progesterone for ten weeks. it minimizes the chance of miscarriage by 50%. >> reporter: she eventually gave birth to healthy twins at age 4 the. and then there's celine who talked about her miscarriage and her ivf atechlts. >> for me to share this, it's about bonding with other women going through the same thing. >> reporter: ivf is costly. it costs an average of about $13,000, not including the medication which can cost up to $5,000 more. a hefty price tag for most american women. >> in the united states, most of the assistive services like ivf,

donor egg, are not covered by insurance. >> reporter: janet jackson has yet to comment on the reports of pregnancy and if so, how. we may never know. >> there's really not a lot that i or anyone else really knows. she likes to keep a low profile. >> women who are celebrities becoming pregnant or having babies in their late 40s or 50, i think the concern is it tends to only provide a glimpse into their situation. >> reporter: with more women delaying childbirth, the fertility industry is booming, now worth 4 billion. an egg donor program advertising accomplished donors to hopeful couples. >> the premier donors get higher fee, it's based on education, s.a.t. scores. when we put up a beautiful donor

who is also smart and has other qualities, we get calls immediately. i call it the immediating frenzy. >> and companies like egg banks, throwing cocktail parties appealing to career women hoping to extend the biological clock. >> will you marry me? >> absolutely. >> reporter: this 29-year-old says she found the love of her life on the bachelor. >> we're going to make some babies. >> reporter: but two years earlier the fertility nurse decided to freeze her eggs just in case. >> it's a huge sense of relief. i felt it would be silly for me not to take the opportunity when it was handed to me. >> reporter: but they worry that women aren't hearing enough about the risk of taking drugs to stimulus their ovaries. that's part of the procedure. >> i would like to see clarity about what it means to go under

it. you are putting chemicals into your body. there are risks. >> reporter: and if a woman can't carry her child, a surrogate may give hope. this 40-year-old endured six rounds of ivf and two miscarriages with no luck. >> there was nothing i could do of myself to make my body do this, to become pregnant. there was depletion financially and emotionally. it felt tough. >> reporter: and the cost of a surrogate would be even more, up to $100,000. but luckily for sara, her 61-year-old mother stepped in to help. >> how would that possibly happen? no offense, but who would do that and what doctor would let them? >> we kept thinking we were going to be stopped. >> nine months pregnant, 38 and a half weeks. >> becoming a surrogate and giving birth to her own grandson. >> how are you doing?

>> feeling good. >> reporter: the road to mother hood later in life isn't always easy. ♪ >> reporter: for janet jackson, millions of fans tonight wondering if she'll be trading anthems like that's the way love goes for quiet lullabys some day soon. >> next, the airline that turned being seated next to a crying baby into something to celebrate. be good. text mom. boys have been really good today. send. let's get mark his own cell phone. nice. send. brad could use a new bike. send. [google:] message. you decide. they're your kids.

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it could save your life. know more. but at this moment, she's fighting a brain tumor. announcer: please take a moment and join st. jude in finding cures and saving children. visit stjude.org. i will admit that even though i have a one-year-old at home, when i get on a plane and realize there's a baby nearby, i often feel unhappy and judgmentl. why are the people you're about to see smiling, then? >> jet blue followed these moms as they flew across the country with their little ones. >> i don't want to be that lady with the baby who is screaming for four hours.

>> i'm most nervous about her having a melt down and having everyone around us wishing we weren't in the plane. >> the airline gaining fans with the newly released mother's david owe going viral. the campaign addressing the worst nightmare of traveling next to a screaming baby. >> welcome abard our special jet blue flight. we know it can be hard on all of us. >> every time a baby cried passengers aboard this flight received 25% off their next trip. in other words, four cries equals one free round trip ticket. by the end of the video, the baby is earning a round of applause and everybody on board a free round trip. >> thanks again to all the moms and the babies on board today for making this flight to special. >> if they gave me free tickets, i would adjust my attitude as well. thank you for tuning in tonight. as always, we're online 24 /7 at

abcnews.com. thank you for watching. good night.

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