S.M.
Hi, Damaria. Honey, don't let your emotions get in the way of your thinking right now. If you KNOW that you conceived before you separated from your husband, then it's almost impossible for you to be pregnant with someone else. Once you have conceived, your body is not interested in becoming pregnant again, and so you are not going to be able to be pregnant on top of being pregnant. It's pretty much impossible (although there was a weird thing on the news recently).
Believe the ultrasound and your own calendar that you keep about your periods. Your body is able to conceive usually only during a 1-wk time frame each month, depending on when you ovulate (produce an egg); usually, you can only ovulate once. Once the egg becomes fertilized, the body goes into pregnancy mode, and is only interested in taking care of the developing baby.
You should have nothing to worry about, if your dates are correct and if what you want is for your husband to be the father of your child.
If you are not correct about the dates and whom you were with, however, then only a DNA test once the baby is born will be able to tell you for sure. If your dates are correct, then the baby has to be your husband's.
The development of the baby at this stage is different than at later stages. The embryo and very young fetus grows through certain definite stages at a well-defined pace; so it's easier to tell at this stage how far along the baby is than at later stages in pregnancy, because after you're 4 months pregnant, babies can grow at different rates depending on how healthy the mother is.
So take a deep breath. Ultrasound tests are accurate.
Peace,
Syl