Coming of Age
The theme that runs through the novel, and which in some way ties all its events together, is Belly's navigation of the maturation process. The story happens the summer Belly turns 16—the age in which she can officially start driving (a feat that represents a slice of the freedom and independence of adulthood). On top of the literal act of growing older (having a birthday), various events and internal changes lie behind Belly's coming of age. She starts attending parties and having romantic interactions with others, and her views on her mother and Susannah also shift, becoming less self-centered and childish and more nuanced and sophisticated.
Secrets
Secrets are described as forming and coming to light at various points in Belly's life. Notable examples are her crush on Conrad being a secret, but eventually coming out into the open in being shared with Jeremiah (as part of their pact of "no secrets"). Throughout the novel, secrets are formed to hide insecurities, crushes, embarrassing facts, and more serious truths, like Susannah's cancer and her divorce from Adam. Even though the people holding onto these secrets try to keep them from coming to light, they eventually do, and tend to bring with them effects that change characters' relationships and interactions.
Summer
For Belly, summer represents a long break from the rigid schedules of school and access to the freedom of seemingly unlimited possibilities. In the summer, she can reinvent herself—pretend to be, for example, the cool girl who hangs out with pretty, popular girls like Taylor rather than the unnoticed girl with "thick glasses and chubby cheeks"—and come close to her dream of attaining the affections of Conrad Fisher. In addition, summer "meant [that she] didn’t have to stay at my father’s sad little apartment…that apartment, it was depressing." Summer is also the only space in which she sees the Fisher family, confining it, and in particular, Conrad, to the realm of an almost fantastical part of the year, which for the rest of the year is inaccessible. For this reason, Conrad, in coming in winter to visit Belly, breaks the barrier between fantasy and reality, becoming part of Belly's reality rather than being confined to the fantasy summer.
Change
With maturation necessarily comes change. Some of Belly's worldviews shift as she experiences new things, but we also see attempts to resist change. One instance of such resistance is Susannah hiding the fact that she has cancer from Belly. In reflection, Belly thinks, "Susannah wanted it to be some kind of perfect summer, where the parents were still together and everything was the way it had always been. Those kinds of summers don't exist anymore, I wanted to tell her." While a drastic change has happened in Susannah—she has become sick and is now dying—she resists and attempts to negate this change to create a pleasant environment for Belly. By doing so, however, she leaves Belly in the dark about Conrad's moodiness, and about her and Laurel's frequent disappearances (for doctor's appointments); Belly remarks that she herself recognized, once alerted of Susannah's cancer, that it would have been better to know than to have been kept in the dark.
Initiation
In the first half of the novel, Belly is initiated into a series of experiences that she has previously not had access to, including her first kiss, her first party, and her first time approaching something like a relationship. These initiations are part of her coming of age and slowly solidify an adult identity for Belly. For example, in a party scene, Cam asks her, "'Do you want something to drink?'" and Belly thinks, "I wasn't sure if I should say yes or not." In these crucial moments of initiation, she makes decisions that will help her define her own identity.
Girlhood and Womanhood
This summer is one in which Belly feels that she sheds her girlhood—in summers past, Conrad and Jeremiah saw her as a "chubby little girl" with "thick glasses," but this summer, she has traded her glasses for contact lenses and notes herself that she has become "pretty." In many ways, Belly's coming of age is accompanied by a physical shedding of girlhood into womanhood. Not only do her body and physical appearance undergo changes that mark her womanhood, but she begins to have experiences appropriate for women and not girls—like going on dates, kissing boys, and having encounters with alcohol.
Mothers and Mothering
The Summer I Turned Pretty dramatizes two different parenting styles through Laurel and Susannah. Susannah is characterized as loving, generous, warm, and stubborn, while Laurel is reserved, rational, calm, and frugal. Despite their differences, the two are best friends. Belly often feels a disconnect with her mother, who she perceives as cold; Susannah steps in as a kind of surrogate mother who provides the warmth and emotional support that Belly does not feel she has from her mother, doing things like sitting down to gossip or give her advice about boys and buying her fancy gifts. At the same time, however, Belly slowly learns of the depths of Laurel's love for her. Laurel, for her part, serves also as a tough voice of reason for the Fisher brothers in the face of Susannah's less rigid parenting style and her dwindling health. When the boys have a physical fight outside the house, it is the firm Laurel who steps in, breaks it up, and drills reason into the boys' heads. Though these two mothers and their respective mothering styles vary greatly from each other, they can be seen as complementary (and perhaps this harmonious match explains why Laurel and Susannah are such great friends): each mother offers what the other does not.
