My morning sickness is at 16 weeks and constipation is killing me. I d…

My morning sickness is at 16 weeks and constipation is killing me. I don't know what to do anymore. The doctor advised me to do an amino test. I'm not sure about the spelling, but it involves putting a needle in my womb to get fluid to check if the baby is Down syndrome affected because of my age and it's my first pregnancy at 37. But I refused the test because I'm not sure if it's safe for my precious one. I'm so stressed; I don't know what to think. What do you think?