I am not myself," writes AK Summers, in her cartoon memoir, Pregnant Butch. An image of her in a spacesuit, in the third trimester of pregnancy, floats above these words, her body egg-like and umbilically attached to a spaceship far, far away. She writes that her partner, Vee, has just bought her a nursing bra, and that her freedom to flout established gender rules, to express her masculinity as a butch lesbian, suddenly seems lost. "There's no use denying it. I am eternal woman," she writes. "I am tears and I am snot. I am anaemic and I am purple veins. I am boobies."

The book, subtitled Nine Long Months Spent in Drag, describes how Summers' masculine identity was challenged by pregnancy, by finding herself in a newly feminine body and a bizarrely feminised world. This is a world in which gender roles are much more closely defined, in which people suddenly feel confident commenting on a woman's body – sometimes even touching it.

Summers is used to feeling like an outsider in a female environment, but during her pregnancy she had the odd sensation of being accepted, even subsumed, into the mainstream. "In some ways, it made my butchness invisible and gave people permission to just treat me as a pregnant woman ... For people who are more comfortable with convention, it was like, 'Oh, you're the same as us, after all!'"

This wasn't an entirely positive feeling. "It was kind of great to feel like, oh, I do belong to this wider world, in this easy way that has just not been my experience across the board. If I was talking to some straight people that I didn't know well, it gave them an opening, if they had kids too. But the flipside was that I often felt I had lost my butchness. I thought, oh, but actually I am still different and my life is different, and if that's not recognised and allowed to be a part of the conversation too, then that stinks."

I meet Summers, 46, in the New York office where she works. She is friendly, precise and slightly hesitant about being interviewed for the first time. Pregnant Butch is her first full-length graphic book. She describes her butchness as being defined by a masculine aesthetic, and an attraction to lesbians who identify as femmes. "Seeing myself in relation to femmes helps clarify that sense of masculinity," she says, which made the changes her body went through in pregnancy even more discombobulating, the differences between her and her femme friends shifting and fading.

The idea for Pregnant Butch came when she was first thinking about having a baby – her son, Franklin, is now 10. She mentioned the idea of getting pregnant to a friend, "And one of her questions was, 'What are you going to look like?' It just popped into my mind that I was going to look like a pregnant Tintin. I drew a little sketch, which I still have somewhere." Did she like that image? "Well, I adore Tintin and when I was a kid, that was absolutely what I wanted to grow up and look like, so I would put it in the pleased column. Mostly, I really didn't want to look like a pregnant woman."

Summers was adopted as a child by a warm, entertaining family and grew up with two brothers in the US state of Georgia. She always had a butch identity, but couldn't express it as a girl in the 1980s. "I didn't know any gay people at all," she says. "At the time, it seemed that I would never be able to reveal myself – that deeply closeted adolescence makes you really fearful."

The only butch woman she knew was a sports teacher, who she says she found repulsive, and who was pregnant during Summers's last year of school. The memory of this woman's distended belly resurfaced when Summers was contemplating pregnancy, reviving adolescent fears that butchness was synonymous with ugliness.

At Oberlin, a liberal arts college in Ohio, Summers came out to her family over the phone in her second week, and started expressing her masculinity. She and a girlfriend constructed fantasies about the feminist collective family they would raise. "A way of fleshing out our lesbian utopian ranks," she writes. This was a half-assed fantasy, says Summers now, but in her mid-30s, when she had been with her partner Vee for three years, the idea of a baby became more pressing.

When Summers visited the doctor, it was suggested that eggs be harvested from Vee, fertilised by an anonymous donor, then implanted into Summers' womb, to prevent any possible legal challenge for parental rights later on. But having been adopted, she wanted to use her own eggs. "I wanted to feel a biological relationship to somebody and that feeling was just as emotional as the discomfort of being a masculine person who was about to undergo pregnancy. It was enough to want to do it for that reason." An old friend agreed to be the sperm donor.

During the weeks beforehand, she worried that her masculinity might preclude it, that there might be some deep correlation between femininity and fertility. "It seems silly now," she says, "but at the time I was just ready to hear, at any moment, that there was something wrong with me."

In fact, she quickly got pregnant and eschewed traditional maternity wear for voluminous trousers, braces and dungarees. Developing a more feminine body was intensely uncomfortable for her. "When you're a butch, you want the way you look to be recognised as intentional," she says. "Otherwise, you just look wrong." At one stage, she looked as if she had a beer belly, "like a schlumpy guy – that wasn't ideal but it didn't feel ridiculous. The thing with feminine dress and the accoutrements of femininity is that they make me feel like I've been dressed up by a vengeful older sister, intent on [humiliating] her younger brother."

By the time Franklin was due, these concerns had receded, replaced by the worry that she would, "fail at childbirth. I had this serious insecurity about my physical courage."

This was related to her butchness, she says, "because I had this idea in mind that 'real women' are actually stronger because they are not tied up in so much fear and insecurity, and can just let go. Not to get too psychoanalytical about it, but that's part of growing up closeted and feeling like you have to present a strong face. Really losing it was a terrifying possibility to me and so I banned my mother from being present – which she ignored."

It was a natural birth and after nine months in drag, Summers now feels the experience enhanced her butchness. "It improved my masculinity because it pushed aside some of the stuff that was really extraneous, unnecessary. Giving a shit so much about what people thought, for instance. Having a kid really clarified that my relationship with him is just a million times more important than my relationship with any number of other people and I could look silly or foolish, and it wasn't going to have a lasting impact. It took some of the fear away.

"There's one thing I would add," she says, "which is that I did actually make it through [the pregnancy] and that's a very butch thing too."

Now she goes to karate with Franklin, "a long-haired boy, who's obsessed with mediaeval weaponry and all that kind of stuff," and shares the parenting responsibilities equally with Vee. Franklin is very accepting of gender differences, she says: "He's coming up in a better time and the logic of doing something because it feels right is very persuasive and natural to him. It's so wonderful to see that, and to feel that – to have a son who's really supportive of his butch mum."