My MIL was very particular about me having enough milk and curd, but my doctor said tablets are also necessary. Usually, calcium starts around the 4th month. Always take the specific brand and dosage that your gynac prescribes for you.
Calcium is typically started from the second trimester because that's when the baby's bone development speeds up. However, please don't start any pills on your own. Let your doctor evaluate your blood tests and prescribe the correct dosage for you.
Apart from eating lots of paneer and cheese, I started my calcium tablets in my second trimester. My gynac told me exactly when to start and which one to take. Definitely ask your doctor about it, don't self-medicate!
For my IVF journey, every single tablet was carefully scheduled. Usually, doctors introduce calcium around the second trimester, near week 14 or 15. Please consult your doctor so they can give you the exact prescription tailored for your body.
Most doctors prescribe calcium after the first trimester is over, around the 14th week. It's a very standard tablet but don't just buy any over-the-counter ones. Ask your gynac in your next visit and they will write the right one for you.