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Eeven as she neared her 18th hour of labor on Jan. 27, Christina Applegate remained focused. Self-conscious about her chest since being diagnosed with breast cancer in 2008 and opting for a double mastectomy, she had kept her hospital gown securely fastened. "I don't let anyone see my top half," says the actress, who has undergone reconstructive surgery. "That whole area," she explains, "has remained very private." That all changed the moment an exhausted Applegate, 39, gave one final push and watched as her daughter Sadie Grace was born. Without hesitation, she let her gown fall off as the nurses placed her baby on her exposed chest. "It was such a liberating and beautiful moment," she says. "I didn't care. It didn't matter anymore. Not only was she the most beautiful thing that's ever happened to me-but she saved me too. I got to abandon resentments that I've held on to in that vicinity for years because [my chest] is where she feels the most comfortable and where she's the happiest."

Now curled up on her plush sectional sofa in the Hollywood Hills home she shares with musician-fiance Martyn LeNoble, Applegate has found her own realm of newfound bliss, less than three years after the toughest battle of her life. Holding her 2-week-old daughter, with LeNoble's arms around them both, the actress is utterly overwhelmed by the sense of love she feels for Sadie. "Everything about her is so amazing. It's just a miracle, and you can't explain it," the actress says. "She's healed me in so many ways. She's just made my life so much better. I've been kind of sad for a long time, and she's just opened my whole soul."

The couple, who got engaged last year on Valentine's Day, were ready to start a family but still surprised to find out Applegate was expecting. "We were actively not not trying," she says, smiling. Her cancer history wasn't an obstacle to conceiving-"I didn't have chemo or radiation, so I didn't have to worry about that," she says-but the pregnancy still posed some risk. Because Applegate is a carrier of the BRCA1 mutation, a gene that increases the risk of developing breast and ovarian cancer, her remission could have been affected by her pregnancy hormones. The star, who is routinely screened for any recurrence, says she decided to leave the fretting to her doctors. "I knew they'd stay on top of it," she says. Still, she admits, "no one told me it would be a problem, which was good, because if you plant that seed in my head, that's not a good thing. Ignorance is bliss."

Applegate took a home pregnancy test last summer after "feeling kind of weird" and quickly shared the news with LeNoble, 41. "I was more in shock, so it took me a couple of days," she recalls. "But then I was unbelievably excited." The actress' joy was short-lived, she says, after she started having morning sickness-and then got bronchitis. "You don't want to be nauseated and coughing up a lung. It was horrifying," she says. "I slept in a dark room for two months, crying, going, 'This cannot be the way this is supposed to be!'"

Fortunately, her second trimester brought a surge of energy and she began exercising and taking spin classes again. "Just trying to keep my blood pumping so that she could get all the benefits of that," says Applegate, who gained almost 40 lbs. during her pregnancy. Yet nearing the end, the actress says she was "beyond ready" not to be pregnant anymore. "I really envy women who are pregnant and look fantastic and they're overjoyed and glowing," she says. "I was not glowing. I was like a miserable, angry person."

And as her due date came and went, that anger turned to anxiety, as she began fearing for her baby's health. "My doctor put it best because he saw me upset and scared. Not about being a parent, but because I just wanted to make sure she was okay," she recalls. "He said, 'Well, Christina, that makes sense. You've been on the other side of that phone call where it hasn't been okay. You have to remember she's perfectly fine, and it's going to be awesome.' And then she came out, and she was awesome. She's perfect. Knock on wood."

When Applegate first laid eyes on Sadie, who weighed 7 lbs., 8 oz. and was 20.4 in. long, "I felt my heart literally open up for the first time and like wrap itself around her. It was profound," she says, looking down at her sleeping newborn. "And I'm more in love with her every minute of the day."

Such joy is a welcome relief after the past three years. "I got slammed really hard and had to go into the mode of getting better," says Applegate. "I think I was devastated, and I was just exhausted by all of it."

These days, she's still weary of the "c-word." "I've had to talk about that word more than I've ever had to talk about a word in my life. After a while, it's just like, there's got to be something else to me." Nevertheless, Applegate discovered that several of her nurses in the maternity ward were unaware of her health history. Says LeNoble: "It was in our birth plan: Don't inform us about breast-feeding, because it's not going to happen." Instead, says Applegate, "you don't know how many times they asked me, 'So, are you going to breast-feed?'" Her response? "I tried not to freak them out and just say, 'No, I've chosen not to.' But then another nurse would tell them [about her mastectomies] and then they'd apologize and feel horrible. And I'd say, 'It's totally fine. You don't have to know my business. Let's just not discuss it anymore,'" she says. "Because I am sad that I didn't get to have that with her, you know? It's the kind of thing I always thought I would have. That special bond that mom and baby are the only ones that can share."

Looking down at Sadie, Applegate takes a moment to breathe her in and then smiles. "But then we do this," she says, kissing her daughter's head and nuzzling her. "And we sleep like this for hours."

By her side throughout this entire journey has been LeNoble, whom she's known for more than 14 years and dated since 2008. "He's my best friend. And now watching him with our daughter, I have fallen even more in love," says the actress, who is keeping their wedding plans a secret. Adds LeNoble: "It's all been amazing. And perfect."

While Applegate has lost half her pregnancy weight so far, she's in no hurry to start working out again, despite Hollywood's obsession with quickly shedding the baby pounds. "Who are these people? Of course, I don't want to live in leggings," says the actress, who will next be seen in the comedy Hall Pass with Owen Wilson. "But I can't imagine working right now, because I never want to leave her for the rest of my life."

The couple also look forward to adding to their brood-someday. "I just can't fathom it happening for at least three, four, five years," says Applegate, who faces a big decision before then: whether to take preventative measures for her increased risk of ovarian cancer. "There are an awful lot of things you can do. You can remove your ovaries, and you can still carry [a child]. My doctors talked to me about it all," she says. "I think we can make sure we have everything proper, one way or the other."

Applegate's mother, Nancy Priddy, has also battled breast cancer-and so Applegate knows firsthand the hereditary concerns when it comes to having children, especially a little girl. "Unfortunately that whole thing is a family affair, and it doesn't necessarily mean that it will get passed on, but you can't help but think that and worry about that," she says, her eyes welling up with tears. "But that's not something we think about. You don't want to focus on it. We're just going to keep her as healthy as possible. I'm more worried about hitting her head when I'm going upstairs or giving her too much or not enough formula. Those are my daily things that I'm learning, to be a mom. I want to make sure I'm doing it all right."

So far, so good: One of Applegate's favorite keepsakes from her own childhood is Dr. Seuss' My Book About Me. "I was 7 years old, and one part asks, 'What do you want to be?' And I put 'Actress, dancer, singer,' and then I crossed that out and I put 'Mother' really big," she says. Applegate gazes down again at her daughter, who is now happily cooing and drinking her formula. "That was the piece of the puzzle that hadn't been there, and now it's there. It's a great feeling of renewal. It's a beginning," she says softly. "She's my beginning."