Pregnant- based on this passage from 'What the Warlock Said' by Busby-Lizzy

When Alec' sister, Izzy, was pregnant, she wanted to keep it a surprise. Of course Magnus knew about it because he could sense the presence of the baby. And when Alec saw Magnus talking to Izzy in the kitchen with a pregnancy test in hand, Magnus had no other choice but to say. "Alec, I'm pregnant. We're going to have a baby." Alec had a headache for the rest of the day because he fell head first as he fainted.

https://m.fanfiction.net/s/9788353/1/What-the-warlock-said

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"Magnus, you don't understand," Izzy hissed. "I can't tell him. He'll flip!"

Magnus rested against the kitchen counter. "Isabelle, dear, he's going to find out at some point. Look at me, I can tell just from being near you."

Izzy gave him a sarcastic look. "You're a warlock. He's an idiot."

"My idiot." Magnus said with a soppy smile. "No, he is pretty obtuse, but you have to tell someone else. Especially Simon, if he's the father."

Izzy put her face in her hands. "By the Angel, I forgot about that. Are you sure? Like a hundred percent sure that it's..."

Magnus grinned. "I'm a warlock, honey. It's my job to know these things. But if you want proof," he snapped his fingers and a small cardboard box appeared. "You can use this."

Ten minutes later, Izzy and Magnus were back in the kitchen.

"How much longer do those things take?" Izzy asked, gesturing to the test in Magnus's hand.

"Patience, dear, it'll show up soon- oh wait, there's something coming up..." he grinned up at Izzy with sharp white teeth. "Congrats."

Izzy paled slightly. "Crap. Oh, crap. Are you sure? Have you done this before?"

"I'm a warlock, we can't have kids. Didn't you pay attention in class? Besides, it says 'two to three weeks' so I assume that's how pregnant you are."

Izzy ran a hand through her black hair. "Great. Alec's going to flip, you know."

"What will I flip about?" Alec said, rubbing his eyes and walking through the door from the bedroom.

The two froze.

Izzy looked at Magnus desperately.

Alec nodded his head at the test in Magnus's hands. "What's that?"

He turned to the coffee machine and started boiling some water.

Izzy and Magnus looked at each other with eyes. Magnus saw the fear in Izzy's dark eyes and rubbed the back of his neck.

"It's a pregnancy test." He said brightly.

At this, Alec turned around, frowning confusedly. "What for?" He asked suspiciously.

Izzy rolled her eyes, despits her panic. "It tests if people are pregnant, genius."

Alec placed the spoon he was stirring his coffee with carefully down on the counter. "And who is pregnant?" His voice had a dangerous edge to it.

Izzy bit her lip, but before she could say anything Magnus took a step towards her brother.

"Me."

Alec rolled his eyes and smiled slightly. "Hilarious."

Magnus showed him the words on the pregnancy test and smiled brightly at him. "Alec, I'm pregnant. We're going to have a baby."

Alec froze, eyes fixed on the test. "Mmnggh?"

"I'm sure this is coming as a surprise, but I did the test and it's positive."

Alec was gradually getter paler and paler. He looked up in mild horror and fainted forwards onto the linoleum flooring.

"When do you think he'll wake up?" Izzy said, placing a cold towel on her brother's forehead. Magnus shrugged. "Who knows, he hit his head pretty hard."

Alec groaned, and the two looked at him again. He propped himself up on his elbows, and blinked. "By the Angel, my head hurts. What happened?"

Izzy smiled at him. "Nothing, you came out of your room and fainted randomly."

He frowned. "No, no, you're pregnant. Oh my God, how did that even happen?"

Izzy froze before realising that he was talking to Magnus.

Magnus shrugged. "No idea. Not that I'm the first warlock to have kids, my friend Tessa does, and I'm sure magic will find a way to do things to make it possible."

Alec flopped back onto the sofa, head aching.

"This isn't a joke, is it?"

Izzy patted his arm. "Afraid not. I was there when the test said positive."

Magnus was getting quite into the idea of having a child, Izzy thought, as he ran through possible baby names (including Maggie, Magnus Jr and Maglec) to an ill-looking Alec. She wasn't sure how long they could keel it up, but she wasn't telling Alec until Simon knew that she was having a kid, at least. And it was quite funny to watch Alec freaking out about a nonexistent child of his.

When the doorbell rang, Izzy exchanged a look with Magnus, who shook his head minutely at her. She rose an eyebrow and grabbed a knife, heading to the door.

She relaxed as she saw Jace and Clary in the doorway. Jace pushed past her into the apartment, and Clary sighed after him.

"He fell over and started talking about Alec?" She said as a way of explanation.

Izzy snorted. "He fainted earlier. He had some... unexpected news." She said, smiling, as they followed Jace.

Alec was now sitting upright, occasionally rubbig his head as if it hurt (and he had hit the floor quite hard, Izzy remembered).

Magnus stood brightly, dropping a wink at Izzy, and addressed Clary and Jace.

"All we need is Simeon to make this party complete. Oh well, we'll start without him. Alec and I have some news."

Jace was sitting on the other end of the couch that Alec was lying on, and looked sideways at his parabatai.

Magnus continued. "I'm having our child!"

Jace started spluttering, while Clary choked slightly.

Izzy hid a smile as Alec looked ill again. To stop herself laughing, she sent a text to Simon asking him to meet her at Taki's. The reply came and she grabbed her bag.

Clary was talking to Magnus about the anatomy and how it was impossible, to which he winked and placed a finger on his lips.

Jace was laughing at Alec, apparently he hadn't paticularly paid attention in class either. Izzy took that as her cue to slip out of the apartment and tell Simon the real news.

The next day, after Alec's headache had mostly subsided, Magnus told him that he wasn't actually pregnant, about which Alec was immensely pleased, and that it had been Izzy's pregnancy test, about which he wasn't, and ended up fainting onto the floor again.