Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

The comic takes place during an in vitro fertilization (IVF) procedure. A doctor is narrating the egg retrieval process, explaining that they are inserting an aspiration needle to extract the ova. But the doctor is suddenly distracted because the eggs come out brightly colored, like Easter eggs. The husband proudly explains that he has been secretly injecting his wife with xanthene dyes (a class of fluorescent dyes that produce vivid colors) for the past three weeks.

The punchline, "Happy Easter!", ties the whole joke together. The humor comes from the absurd and medically horrifying idea of someone secretly dyeing their partner's eggs inside her body to create real-life Easter eggs. It is a dark play on the double meaning of "eggs" -- the biological ova being retrieved during the IVF procedure and the colorful decorated eggs associated with the Easter holiday.