You are 18 weeks along, and all appears to be normal. You really shouldn't worry. Because we can't just peek into our wombs, fetal development is based on a healthy heartrate, intermittent scans (ultrasounds), and by your practitioner feeling around (palpitating) your abdomen for your uterus. Also, the mother's health is also taken into consideration. You don't smoke or drink, you do receive prenatal care, and appear to be a concerned mommy to be...you are doing well.
Stressing too much, too often isn't good for your well-being, but it will not keep your baby from growing. Stress isn't good for the baby, either, let your babe be relaxed. ;) Fetuses are kind of like little parasites, they will take what they need, whether the mother is stressed or not. An example, when your baby needs calcium for it's skeleton, and you are not taking enough calcium from your diet to supply it, the baby will take the calcium from your skeleton...baby comes first, even before it is born...heh heh.
18 weeks is still early for you to identify your baby's movements if this is a first pregnancy. At this point, you probably have felt it move, but it resembles gas or bubbles in your gut area, the only difference is that you just never pass the gas (fart) heh heh heh...that's how I was. I didn't feel an actual "kick" until I was nearly 23 weeks with my first pregnancy, and even then, I didn't feel the movements everyday! I am currently 36 weeks and 4 days.
If your midwife had any concern about your pregnancy at this point, it would be brought up to you. You are close to getting your second ultrasound scan, usually around 20 weeks, and this is the big one that will check fetal growth, and you may even get to find out what you are having (boy/girl).
Keep healhty, eat a balanced diet, drink lots of water, and relax...sounds like the pregnancy is progressing as it should. Congratulations....be happy!
P.S. You are not alone with worrying about your baby's development, I still worry every now and then, but I rely on my pregnancy being healthy so far, which is all we can do.