Can you have pregnancy symptoms 12 days before missed period?

Unlikely: Conception, as defined by the generation of a fertilized ovum, occurs within day or so of the sex. However, it takes about 3 weeks for the embryo to make enough HCG to produce a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. If the result is negative repeat the test in 4-5 days. Use first morning urine and follow instructions for the test carefully. If you do not wish to be pregnant, use contraception all the time, every time. You may consider implanted contraceptive, or IUD. Practice safe sex, if you have sex. Get HPV vaccine

No: The symptoms of pregnancy are triggered by the same hormones that turn a urine test positive. It takes time for the conceptus to travel to the womb, implant & begin to create enough of these hormones to either cause symptoms or a positive test. For most this will be about the time you miss a period. You may be anticipating symptoms & your mind can do confusing things.