I don’t think the start date of your analysis matters to 1986 or 1990. I would want to run the data by someone who deals in Mexican demographics. I wonder how the entity that produced this data handles non documenteds leaving the country.
Interesting that Mexico has 2.5 million births compared to USA 3.9 million births. Also note that the # of births is declining like it is in the USA.
I just went with 1990 as the baseline as it was shortly after the amnesty and was the earliest data they had an overall population for. And yes, the birthrate was phenomenal.