Our child was born in the US in February. Her mom is Thai but a legal permanent resident in the US. We're traveling to Thailand for several months in January-March to see relatives but aren't sure which passport our baby should show at the airports each way. If we show the US one both ways, do we just go to Thai immigration to get an extension for after the first 30 days? (I have a US passport but am only staying under 4 weeks. Yai is coming back with mom and babe.)

We'll be primarily in Chiang Mai if that makes any difference.

Thanks for any help.

Use US passport to exit US, Thai passport to enter/exit Thailand, US passport to re-enter US. You will need to show both passports at airport check-in. Entering Thailand on Thai passport avoids immigration requirements that would otherwise apply entering on (foreign) US passport, and can stay as long as wants to.

I concur with Thaiphoon. I have two passports. I do the same. There is nothing illegal (from either the U.S. or Thailand) in having more than one passport. For simplicities sake, do what he says. To leave and enter the U.S. use the U.S. passport (good luck using the Thai passport--it is essentially useless in terms of coming into the U.S.), entering and leaving Thailand, use the Thai (shorter lines for Thai passport holders, which you should also benefit from because you can go in the group with your family since there are two Thai passport holders).

As said try to use as above. But you may have to explain why there is no exit card in Thai passport (obviously not in Thailand before) so it would not be a big deal to enter on US if it saves having an issue with Immigration. No overstay is charged for children so there would not be any need to obtain extensions.

Second issue is US. That requires the use of US passport by any US citizen for entry/exit. Thailand does not have that rule.