When my first daughter was a year and a half old I replied to an ad on Craigslist from a young woman, I'll call her Y, looking for work as a part-time caregiver. I was newly pregnant with my second daughter and in need of a few days a week to write, shop and collect my sanity.

When Y showed up at my front door for the interview, I was immediately put at ease -- warmth and sincerity radiated from her like the steam from my (unfortunately) decaffeinated latte. Within 10 minutes of our first meeting, she had gained the trust of my toddler and I was ready to hand her the keys to my house.

The next few months were rather difficult for my family. My second pregnancy was accompanied by a myriad of problems including a brutal bout of prenatal depression and preterm labor that left me on bed rest for the last two and a half months. During this time I was forced to lean on those around me. Y proved herself to be a steady constant for both my daughter and myself, she was someone with endless patience for the trying terrible twos and the energy to do the activities I couldn't.

When the baby was born, Y proved even more invaluable. Given that my time and attention was split between the two children, and Y mostly babysat while my older one was in morning preschool, the baby and Y formed a relationship that could only make a mother jealous. When Y would climb our front stairs, the baby's face would light up as though Elmo himself had come to pay her a visit. She would reach for Y and giggle as Y smothered her cheeks with kisses. Mommy who?

It was obvious from the beginning that Y was queer. She dressed in typical San Francisco "butch" fashion and was always open about her social/sexual life. I loved this about her. While I am a rather feminine, girly-kind-of woman, I was excited to have a different example of femininity to show my girls. Being a woman did not have to mean growing your hair long, dressing in skirts and playing princesses; here was a model of a woman who defied all that.

One evening, nearly two years after I first hired her, Y and I sat chatting at the end of a workday, as we often did. While the girls played at our feet, she told me the news; Y had decided to start hormone therapy and begin the process of transitioning to a man.

My initial reaction could be summed up with three words: confusion, bewilderment and skepticism. Although I consider myself to be quite progressive and supportive of the LGBTQ community (both of my maternal grandparents are gay) I was pretty certain that this was the wrong decision. "Are you sure?" I asked. "Is it because you are trying to distinguish yourself from your identical twin sister?" "There are easier ways to not have a period anymore." "Could this be a fad?" Unsurprisingly, Y was upset with my inability to provide unconditional support.

While I tried to disguise my questions as legitimate concerns, in reality I was scared. How would she change? What did this mean for our relationship? How would this affect my girls? They already had one straight male caregiver (my husband), but now, who would provide them with that other side of femininity? I worried about losing a friend and a piece of my family.

Y's identical twin sister almost exclusively dates transgender men. Although many research studies have established a correlation between the sexuality of identical twins (these twins are more likely than not to be of the same sexual orientation), little research has been conducted on gender identity and identical twins. There are a handful of cases where both siblings in a set have transitioned to the opposite gender, and a handful of other cases where just one sibling transitioned. Genetic composition may be playing an important role here, but for now, it is too early to tell.

Whether gender identity is genetic or not, there is a bounty of evidence pointing to a biological connection. With the use of MRI brain scans, doctors have found that both male-to-female (MtF) and female-to-male (FtM) transgender persons show significant neurological differences from the non-transgender. When shown erotic images, MtF subject's brains respond closer to that of a biological female than a biological man. The FtM subjects showed brain responsivity that differed from both biological men and women.

Y's twin sister, N, has called Y "Brother" since they were little, and she was not surprised by his decision. Similarly, my older daughter did not flinch when Y changed his name and cut his hair short. To Elana, Y had never been a girl, yet somehow not quite a boy. Before I knew of Y's gender identity, I would argue with my toddler when she referred to Y as a boy. "No," I would reply, "Y is a girl." Even at three years of age, Elana would look me dead in the eye and tell me I was wrong (though she did use female pronouns when referring to him).

Over the next year and a half, Y experienced many physical and emotional changes. His weekly testosterone therapy changed his body quickly, his hips narrowing, upper body muscles growing, voice lowering, and rapidly growing facial hair. The emotional changes also started soon after the introduction of the hormones, and this is what I feared most. The testosterone left him more masculine. His perception on everyday issues became more concrete, while he grew less emotionally involved with drama that surrounded him. In addition, his confidence, as well as his sex drive, skyrocketed.

However, there was one very important aspect of Y that never changed -- his close relationship with my children. As a caretaker, he continued to be fantastic, loving my two girls as though they were his little nieces and he was their uncle. Their admiration for him never faltered, he was exactly who he always was -- fun, creative, and loving.

Having a non-female caregiver gave my daughters additional benefits. Since my husband, their father, works long days, it was refreshing to have a masculine figure helping in their early childhood. While I know nothing about cars and bugs, and tend to lean toward the more traditional girl clothing, Y was there to point out fire trucks, help them search for worms at the park, and dress them in bib overalls. He also braided their hair, taught them how to bead necklaces and bracelets, and threw spontaneous dance parties. It was the exact same things that he had done when he was a woman.

Y's transition changed me too. Watching Y's struggle with weekly hormone therapy, decide when to come out to his family, friends, and employers, and select the appropriate public restroom, transformed my beliefs on gender identity. Even though I always supported the notion that people could be born into the wrong gender, I now view gender as more fluid -- if there is a spectrum for sexuality, maybe there is also one with gender. I started making sure that I approach gender more sensitively with my own girls, allowing them to tell me who they are.

Because of Y's influence in our lives, I made the conscious effort to choose gender-neutral toys and clothing. When the decisions were still mine to make, I purchased balls and blocks, in lieu of Barbies and Hello Kitty, and opted for brown and green shirts, instead of pink and purple. Once the girls began to exert their own unique fashion sense, I encouraged them to select their own clothing, making sure that they had a variety of colors and styles to pick from.