With the help of doctors at the House Ear Institute, she attempted to save herself and her child.
Pregnancy causes many strange symptoms, but at the start of Siwek's third trimester, she lost hearing in her left ear. A doctor noticed even more.
"And he says, 'Did you know that your left eye is actually open a little wider than your right and isn't blinking the same?'" said Siwek.
An MRI confirmed the worst: a benign brain tumor called an acoustic neuroma was pressing on her brain stem.
Siwek lost her first baby to pre-term labor. Now she didn't know if she'd be able to carry this one all the way.
"All I knew was that whatever I was going to do I needed to find a surgeon who would get me back to not only getting this tumor out but getting me back to 100 percent to being a good mom," said Siwek.
The Milwaukee mom-to-be found Dr. Rick Friedman at the House Ear Institute. He saw the tumor was tangled in the nerves that affect hearing, sight, balance and facial movements.
"It was really pushing her brain way past the midline. She couldn't walk, she was having double vision and headaches," said Friedman
And that's what she struggled with during the rest of her pregnancy. She cried tears of joy at Parker's birth, but Siwek was distraught.
"I couldn't pick him up or do any of the things you think a normal mom would do," said Siwek.
In the back of her mind, she worried she might not survive the delicate surgery, or that she would be disabled.
"So there were periods when she was pretty down. I could imagine she didn't think things would be much better," said Friedman.
With the help of another surgeon, Friedman painstakingly removed the tumor from the tangle of nerves that surrounded it. One bad move and things could have gone terribly wrong.
"The risk of stroke, brain stem injury, permanent facial paralysis. There are devastating possibilities, absolutely," said Friedman.
Following the marathon procedure, home video shows Siwek's vision, balance and strength returned.
"She looks great," said Friedman. "Her smile's perfect, her balance is better, her vision is better. She looks great."
Siwek permanently lost hearing in her left ear, but she says that's a small price to pay for the family she's always wanted.
"I may never be 100 percent physically, but I just have a whole new lease on life," said Siwek. "If that's one positive I'll never take anything for granted again."
An acoustic neuroma is a non-cancerous tumor of the nerve that connects the ear to the brain. Symptoms include unexplained hearing loss in one ear, ear ringing and dizziness.