Author's note: While I don't want this to happen in canon, the idea wouldn't leave me alone. Hope you enjoy! And warning for childbirth.
When Mary started feeling off about six weeks after John's unexpected visit, she figured she was getting the flu and brushed off the rampant nausea for a few days until a visiting Jody cornered her outside the bathroom with a pregnancy test. When the results came up positive and the doctor confirmed the news, she broke the news to her very surprised - but also excited, once the shock wore off - boys. Needless to say, the two doted on her throughout the pregnancy, waiting on her hand and foot. When they weren't around, someone else - like Cas, Jack, Rowena, or Charlie - was there to help. None of them were taking any chances. And when her Halloween due date came and went with nothing happening, a frustrated Mary sat down and cried for a little while. "Sam was two weeks late," Dean pointed out, earning a glare from his mother.
"Don't remind me," she snapped.
"You want us to get you anything?" a concerned Sam asked her.
Mary continued to glare at her boys. "Stop hovering." A beat later, she added, "Please." She wandered around the bunker all day, Jack trailing behind her (he was the only one she was nice to once she entered the last weeks of her pregnancy) in case she needed anything. Trying to sleep proved impossible and she gave up around four in the morning when she felt the first twinges of pain. Trying to discern whether she was experiencing Braxton Hicks contractions, she timed them on her phone for a little while. It wasn't until the pain intensified that she finally admitted she was in labor and went to find the boys. They looked up expectantly when she waddled into the kitchen. "This is it. It's happening," a beaming Mary informed them.
Dean froze for a few seconds while Sam rushed to her side and hugged her as best as he could with her stomach in the way. "You in any pain?"
A contraction started to build and Mary nearly doubled over in pain but Sam helped her stay upright and grabbed her hand, letting her squeeze his until it was over. "It's going to get worse so buckle up." She couldn't wait to finally meet her daughter, this third little person that she and John created together.
"Do you need me to get you anything?" Dean finally asked.
"Not right now, sweetheart, but thank you." Mary sat down and tried to relax. True to form, Sam and Dean refused to leave her side for anything. They timed her contractions and offered their support any way they could, which she'd thank them for later when she could think straight again.
After laboring all day, Mary's water finally broke around nine that night and things ramped up. Contractions got closer and closer together, which terrified the boys slightly but they made sure to mask their fear in front of their mother.
When it came time to finally deliver the baby - with Jody's help (Alex would have come but unable to call off work because of an emergency, their back up plan was instituted) - Sam and Dean held Mary's hands and offered her words of comfort when the pain got to be too much and she wanted to give up. "You're so close, Mom. She's almost here," Sam soothed.
"I don't want to do this anymore," Mary sobbed before crying out in pain again.
Dean really wanted to punch something or somebody but he settled for trying to talk her down. "It's a little too late to get out of this, Mom. But you are doing this, and you have been all day. You're going to meet your baby soon. And I bet she's really fucking excited to meet you too." He kissed her forehead.
"Fuck," Mary moaned. She screamed and leaned forward to push, finally delivering the baby ten minutes later.
"Congratulations!" Jody cheered as she cleaned the baby up. After Dean cut the cord (Sam's hands were shaking too much), she handed the baby over to Mary. "Here's your mommy, little one."
An exhausted Mary immediately fell in love with her little girl the instant she laid eyes on her. "Hello, baby girl. I'm your mommy, and these are your brothers." She looked up at Jody. "Thank you so much for everything these past nine months."
Jody shrugged. "No big deal."
"She's so tiny," an awed Sam whispered as the baby let her displeasure known and started crying.
"You were that tiny once," Dean murmured, bumping into his brother with a grin on his face. Sam bumped him right back.
"She's perfect." Mary started rocking the newborn back and forth, trying to calm her down. She finally quieted down a few minutes later.
Jody smiled at them and then left the room to give the Winchesters some privacy. "Have you come up with a name yet?" A curious Sam stared at his mother hopefully.
Mary nodded and then looked up at them, an exhausted grin on her face. "I have. It's a name your father wanted for either of you if you had been a girl. Meet Alice."
"And her middle name?" pressed a grinning Dean.
"Johanna, after your father. I was thinking about Johanna as a first name for a long time but it just doesn't feel right to me. And now that she's finally here, she's definitely Alice."
"Alice Johanna Winchester." Sam nodded his head in approval and beamed at her.
Mary looked down at her now sleeping daughter and sniffled. "Your daddy would have loved you so much, Alice. But you have two very overprotective big brothers who would go to the ends of the earth for you and a lot of aunts and uncles who are probably dying to meet you. I love you too so don't ever forget that." She kissed the baby. Then she looked up at her now middle child. "What time is it?"
Sam got out his phone. "It is 1:04AM on … November 2nd. Holy shit." He hadn't even been paying attention to the time before so the fact Alice had come along on this day filled with pain was a shock to the system and also meant so much.
"Well, hey, we finally have a good November 2nd memory. It's about time." It wouldn't erase the trauma of the day but Alice's birth would go a long way towards healing it just a bit, just like Mary's resurrection had. Dean was grateful for that.
Exhausted after spending the entire day in labor, Mary handed Alice over to Dean and fell asleep within minutes. "Hey, you. You're adorable." Dean still couldn't wrap his head around the fact that he had a little sister now - Mary dropping the bomb that she was pregnant was still one of the biggest shocks of his life (Sam's too). The forty and thirty-six year age differences were also a little surreal but they'd get over it. The baby didn't stir, which he was a little thankful for because his mother desperately needed sleep. And after knowing her for less than an hour, Dean loved Alice so much already he'd kill anybody for attempting to hurt her.
Sam watched the two of them together and it took his breath away. "You ever going to give her up?" he teased.
"You can hold the baby when I'm done with her," Dean teased right back before finally handing the baby over to his brother about ten minutes later. Sam panicked a bit initially but he got the hang of it pretty quickly.
After Mary had recovered the next morning, Jody snapped the first picture of the four Winchesters together and immediately texted it to her girls, Donna, and the rest of their found family (much squealing ensued, with multiple people promising visits). "I am so excited to get to know you, Miss Alice," Mary whispered after she finished feeding the newborn again. Although she still missed John so much it ached, she was so thankful for their sons and daughter - she loved them with every fiber of her being.
Alice would never get a chance to know her father but she'd grow up up surrounded by love and with a stable support system.