Several factors can influence fertility, including:

1. Age: A woman’s egg quality and quantity decrease with age. Men’s sperm quality also declines, albeit more gradually.
2. Lifestyle Choices: Smoking, alcohol, and drug use negatively impact fertility in both genders.
3. Weight: Being overweight or underweight can disrupt hormonal balance and ovulation.
4. Environmental Exposures: Pesticides, heavy metals, and radiation can affect reproductive health.

Chronic Medical Conditions: Diabetes, thyroid disorders, and autoimmune diseases may contribute to infertility.

Common Causes:

1. Age:  Fertility declines significantly after the age of 35 in women, and in men, it significantly decreases after the age of 40.
2. BMI: Obesity impacts the ability for conception. 
3. Lifestyle Factors: Smoking, excessive alcohol consumption, obesity, or extreme stress.
4. Unexplained Infertility: In about 20% of cases, the exact cause remains unidentified.
5. Genetic Factors: Conditions like Klinefelter syndrome, Turner’s syndrome, Fragile X syndrome etc.
6. Immunological factors: Autoimmune conditions or abnormal immune responses that attack sperm, eggs, or embryos can lead to infertility. Conditions like antiphospholipid syndrome or abnormal natural killer cell activity are examples.

In Women:

1. Ovulatory Disorders: Conditions like polycystic ovary syndrome (PCOS) or premature ovarian insufficiency.
2. Uterine or Cervical Abnormalities: Fibroids, polyps, or structural abnormalities.
3. Fallopian Tube Damage or Blockage: Often caused by infections, endometriosis, or pelvic surgeries.
4. Endometriosis: Tissue similar to the uterine lining grows outside the uterus, impacting fertility.
5. Hormonal imbalance: Certain hormones like thyroid stimulating hormone & prolactin may interfere with conception.

In Men:

1. Low Sperm Count or Poor Sperm Quality: Affects the ability to fertilize an egg.
2. Hormonal Imbalances: Reduced levels of testosterone or other hormones.

Varicocele: Enlarged veins in the scrotum can impair sperm production.

Fertility treatments are recommended for individuals or couples who:

• Have been trying to conceive for over a year (or six months for women over 35).
• Have known medical conditions affecting fertility, such as PCOS, low ovarian reserve, low sperm count, certain genetic conditions etc.
• Experience recurrent pregnancy loss.
• Are planning parenthood later in life and wish to preserve fertility.
• Have undergone cancer treatments or other medical interventions that impact reproductive health.

Timely intervention is crucial for optimizing success rates. You should seek help if:

• You’re under 35 and unable to conceive after one year of trying.
• You’re over 35 and unable to conceive after six months of trying.
• You’ve experienced two or more miscarriages.
• You’ve been diagnosed with a condition known to affect fertility.
• You’re interested in fertility preservation.
• You’re undergoing cancer treatment.