Twins provide a unique context for studying the interaction between genetics and the environment on human traits. Monozygotic (MZ) twins share nearly identical genetic material, and dizygotic (DZ) twins share about 50% are influenced by environmental factors, especially during prenatal development. Chorionicity, or whether twins share the same placenta, plays a key role in shaping their in utero environment, influencing risks such as unequal nutrient distribution and conditions like twin-to-twin transfusion syndrome (TTTS). Postnatally, shared environments and lifestyles can further affect twin development, although their susceptibilities may lead to different outcomes. Factors like maternal nutrition, exposure to toxins, and prenatal care are critical, particularly in the context of neural tube defects (NTDs), where both genetic and environmental influences, such as parental occupation, maternal obesity, and folic acid deficiency, play a role. Understanding these risks is vital for preconception counseling and effective pregnancy management to optimize outcomes for twins.