I don't have my period yet!
"I'm a 13-year-old female who has not had her period yet. All my friends already have their first period. Is there something wrong with me?"
Amenorrhea is defined as the absence of menstruation. It may be classified into two types - primary and secondary. Primary amenorrhea occurs when one has not had the first menses by the age of 16 years. Secondary amenorrhea occurs when one has had menstruation before, but has not had one for at least 6 months. Oligomenorrhea refers to infrequent menstrual cycles occurring less than 6 to 8 cycles a year.
More specifically, primary amenorrhea is defined as "the failure to initiate menses by age 14 in the absence of secondary sexual characteristics or the absence of menarche by age 16 regardless of the presence of normal growth and development of secondary sexual characteristics." 
The science behind
To understand how menstruation occurs, we must first learn about some basic science and body parts involved. The hypothalamus, which resides in your brain, sends signals called the gonadotropin-releasing hormones (GnRH) to a gland called the pituitary gland. The pituitary sits at the base of your brain just behind your nose. When it receives signals from the hypothalamus, it then sends its own signals (called the luteinising hormones (LH) and follicle stimulating hormones (FSH)) to the ovaries which then produces sex hormones oestrogen and progesterone. Oestrogen and progesterone go on to regulate your menstrual cycles. This is known as the hypothalamus-pituitary-ovarian (HPO) axis. The photo below illustrates this cycle.
Image credits: Dr Susan Caldwell
Therefore, your menstrual cycles are dependent on the level of sex hormones in your body, which depends on the signals sent by the pituitary gland and the hypothalamus sitting in your brain. Causes that affect any of these organs have potential to affect your menstrual cycles.
The most common causes of primary amenorrhea includes structural abnormalities, hormone imbalances (e.g., FSH), underlying conditions such as polycystic ovary syndrome (PCOS) and anorexia nervosa. Structural abnormalities include an imperforate hymen, vaginal defects, or abnormalities with the uterus and/or ovaries. PCOS is a condition which causes amenorrhea when the ovaries produce excessive androgens. A genetic condition to look out for is called Turner's syndrome. It usually characterised by short stature, webbed neck, as well as a broad chest with widely spaced nipples.
Photo credits: MSD Manuals
When a patient first presents with secondary amenorrhea, the doctor will usually do a urine pregnancy test to first rule out that the patient is not pregnant. This is one of the most common causes of secondary amenorrhea. It is also important to note that if you are currently breastfeeding, you might have irregular menstrual cycles too; you might not have your first period for at least a few months. If the patient is not pregnant or breastfeeding, then the doctor might recommend a workup to investigate the causes. They can be categorised according to the part of the HPO axis. Causes include a brain tumour or infection, medications, autoimmune causes (e.g., thyroid disorders), complications relating to childbirth, and PCOS.
Gasner, A., & Rehman, A. (2021, January). Primary Amenorrhea. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK554469/