I've played with the timeline a bit. Here's what I've established: Cell appears, they train in the time chamber, go looking for the androids before Cell can absorb them, fail and then are given a week before the Cell Games in which only Vegeta, Trunks, and Piccolo use the time chamber again. Gohanna and Goku do not.
Mother," Gohanna said, voice trembling, "I need to talk to you."
Chichi paused at the sink, surprised and full of trepidation at what could possibly prompt such a tone from her daughter. She rinsed her hands off and joined her daughter in the living room. She couldn't help the sigh of pleasure when she sat down; she had forgotten how exhausting being pregnant was!
Gohanna couldn't meet her mother's eyes; to Chichi's observant eyes she looked rather frightened and ashamed. Chichi felt her worry rise; what could possibly make her daughter look that way?
"Honey, what's the matter?"
Gohanna still wouldn't look at her mother, Chichi wasn't having any of that, and she reached out and gently lifted Gohanna's face, looking her in the eyes she said, "I don't know what is going on and you are starting to worry me. What is it?"
Gohanna knew if she didn't say it now she would lose her nerve. It had already taken her over a week to gather the courage necessary to talk to her mother. She drew in a deep breath and blurted out, "I'm pregnant!"
Chichi froze, surely she hadn't just heard… "What?"
Gohanna let out a ragged breath, "I'm pregnant."
Chichi still felt rather numb with shock, "How? You're eleven!" She was struck by another thought, "Who is the father?"
Gohanna flinched and took a deep breath to settle her nerves. "Android 17."
Chichi gave her a confused look, clearly not understanding the implications. "I thought the Androids were evil and that Cell killed them?"
Gohanna swallowed, "Android 18 doesn't seem so bad and yeah Cell killed them, but the dragon balls brought them back. I killed Android 17 again, though."
Chichi blinked, stunned that Gohanna had apparently hunted down and killed the android a second time, that didn't sound like her. "Why would you do that?"
Gohanna still felt scared of her mother's reaction and on some level she actually felt relieved that her mother seemed to have forgotten about what she had just learned. Another part of her was annoyed; it was hard enough to talk about without having to spell it out for her. She wondered idly if looking at her mother long enough would be enough to get her mother to magically understand. In the back of her mind she sensed that Piccolo had arrived, sensing her agitated emotions. He was close enough to hear everything, but far enough away that they would not be able to see him.
Chichi felt a dawning realization and her trepidation tripled. She spoke, voice quiet and almost begging as she said, "Why did you kill Android 17?"
Gohanna knew now was the time, she had not counted on telling Piccolo so soon. She had hoped to put off that particular conversation for several more weeks. She knew he would want to know why she hadn't told him right away. "Mom," she paused, "you know after training in the time chamber we split up to try and find the androids before Cell could absorb them. I didn't tell anybody, but," she took a breath, "I found Android 17. He thought it was funny that I would fight him and he said he was going to 'put me in my place"' she said bitterly, at this point deliberately ignoring her mother's look of dawning comprehension and the way Piccolo's ki flared in anger.
"He said that he hadn't 'gotten any' since before Dr. Gero changed him and 18. He said I was close enough to a grown woman—" she cut herself off abruptly. "I'm not saying anything else about it, but you understand now, right?" Gohanna didn't wait for either Chichi or Piccolo to answer before powering on. "I wasn't strong enough then, but I used my hidden power against Cell. I knew I was strong enough now. And I know that killing is wrong, but I don't regret what I did to Android 17. He more than earned death. And I was the only one who could do it."
Chichi's eyes were filled with tears, "My baby was raped?" she whispered.
Gohanna didn't look at her mother, she would have nodded, but she knew Piccolo was waiting on an answer as well. "Yes." She choked out, surprised at how much it hurt to admit. She was surprised to find herself pulled into her mother's lap as the woman tried to comfort her child who had been hurt so badly. As Gohanna started to break down in her mother's arms she felt Piccolo's ki flare again and she knew he had sense the second ki signature in her abdomen, the only reason she had known about the baby.
She decided that talking to him could wait. She needed this release; she had needed it for two-and-a-half-months, ever since that awful day. And with that she let go and sobbed in her mother's arms for the first time since she was four years old.