The recommendation to wait is precautionary, not based on evidence of harm. No study has proven that tattoos cause pregnancy complications, but no study has proven they are safe either. When it comes to pregnancy, the medical community generally takes a "when in doubt, wait" approach.
The primary concern is infection. Getting a tattoo creates an open wound, and any open wound carries a risk of bacterial infection. During pregnancy, your immune system is naturally modified (it downregulates certain responses to prevent rejecting the fetus), which can make infections slightly harder to fight. A serious skin infection during pregnancy could require antibiotics and, in rare cases, could affect the baby.
The secondary concern is tattoo ink composition. Tattoo inks are not regulated by the FDA for injection and can contain heavy metals, plasticizers, and other chemicals. Whether these substances cross the placenta in meaningful amounts is unknown. For other products that come in contact with your skin during pregnancy, see our essential oils guide.