The medical reasoning behind the universal advice
See [DAN's position statement on pregnancy and diving](https://dan.org/health-medicine/health-resources/diseases-conditions/pregnancy-and-diving/). Two specific concerns: (1) nitrogen bubbles forming in fetal tissue during ascent — the fetal lung doesn't filter nitrogen the same way the maternal lung does, so the safety margin is unknown; (2) the first-trimester risk of fetal malformations during organ development. Most agencies extend the no-dive recommendation through the entire pregnancy out of caution, not because of equal risk at every stage.
What you can still do — and the safe alternatives
Snorkeling at surface — generally considered safe (it doesn't expose tissues to nitrogen partial pressure beyond what you already breathe). Gili Air has 5+ shallow snorkel sites accessible from the beach, no boat ride needed; turtles often appear in 2–3 m of water. Freediving is also typically advised against during pregnancy due to the breath-hold pressures involved. The friendliest in-water activity: a calm float at Hans Reef from the beach with mask + snorkel.