Slow hcg

Seeing if anyone else is in my position. I am estimated at 3 weeks 4 days solely on my hcg levels. My confirmation was on Wednesday with an hcg level of 16. My other test was Friday and my levels has only gone up two levels. (18) doctor isn't concerned because the number is still rising. He diagnosed me with slow producing hcg. Has anyone else had this issue? When I was pregnant with my daughter urine tests finally turned positive a week shy of 3 months pregnant. I think I'm farther along then they say and it's stressing me out that I wont know for sure for 5 more weeks how far along I am. Just hoping someone else has the same problem so I'm not freaking out so much?