I think we are beginning to build up a case that suggests Thailand may be no greater risk than many other countries pregnant women are visiting.
1] The reported cases in Thailand in the first 9 months of 2016 are 349 (assuming since January includes January.)
2] The Thai population is over 68M
3] The level of non-reporting is not known
4] Foetuses are more susceptible closer to conception than later (my assumption, see Figure 3). However experts don't know whether you're at higher risk if you get infected during a particular trimester.
5] A model using data from a Zika outbreak in French Polynesia estimated the risk of microcephaly in children born to mothers who acquired Zika virus in the first trimester to be 1%
6] With 4], later in pregnancy might be safer than indicated by 5]. However microcephaly is not the only birth defect to consider, though it seems the others are generally less severe (eg impairment of hearing and/or vision) though sometimes more severe (eg fatal).
7] On the other hand most babies born to pregnant women who get Zika appear to be healthy
8] The model may not be accurate or may not otherwise apply to Thailand. Experts don't know how likely your baby is to have birth defects if you get Zika during pregnancy.
9] CDC do not at present have a watch, alert or warning for Thailand
10] Travel Health Pro has:
This country is considered to have a high risk of ZIKV transmission. Increasing or widespread transmission has been reported. Pregnant women are advised to postpone non-essential travel until after pregnancy. Details of specific affected areas within this country are not available.
Judging by other usage of "non-essential travel" in health and safety warnings, coupled with no emboldening and no "very strongly" or "strongly" in front of "advised", this is barely more than a routine caution - almost obligatory from such a body and more akin to telling a child to be careful when crossing the road, rather than not to stick metal objects into power sockets.
CDC's wording is an even milder consider postponing:
women who are pregnant or planning to become pregnant should discuss their travel plans with their doctor and consider postponing nonessential travel to Thailand.
11] A combination of the above would suggest to me odds of around 1 in 100M per annum and better than that with proper precautions (DEET, no perfume, covering up at dusk and dawn avoiding areas of standing water, trying to stay indoors and high up, nets etc). Much longer than the chances of winning the lottery.
12] You will presumably be in Thailand for weeks rather than long-term, hence limit your exposure
13] If Aedes aegypti breeds like the malarial mosquito (malaria risk in Thailand estimated at 1:50,000) the rainy season is the time of highest risk, and that for Thailand usually ends in November, so you would be there after that.
HOWEVER, you must at least listen to what your doctor says as your medical history is relevant (eg will this add to a stress issue).
To take your questions in turn:
When during pregnancy will Zika cause harm to baby?
There is risk throughout pregnancy (see Comment below).
Do 100% of all bites lead to Zika?
Definitely not (eg there are other mosquitoes and insects that bite).
Is it likely we have a immunity?
My guess is no, but it does effect some people more than others. Once you've been infected with Zika, you're likely to have immunity from the virus but if so, how long for is not known.
are not all mosquitos carriers?
Not all mosquitoes are carriers. Not only does it depend on their species but also their gender.
See also:
Is there a good and objective resource online about Zika virus?
and
How to avoid the contact with the mosquito that carries Zika Virus in Tropical countries
