Your baby weighs a whopping 5 ounces by down and is edging up to the size of a grapefruit. That is noticeable, and your belly is protruding by now too. If you can peek at them on an ultrasound, you will see those making breathing motions, which are just practice, since there is no air yet. They get their oxygen through the umbilical cord from you. (This is why you need to stay away from smoking!) They can wave and suck their thumb and do acrobats while they still have room. Their heart beats at 140-160 beats per minute; this is normal and twice as fast as your own.
Your body continues to expand. If you are a first time mom, this may marvel you as you watch week by week the changes you endure. Some women are predisposed to having stretch marks, unfortunately. As unsightly as they are, think of them as a badge of motherhood. It is your body stretching to accommodate the growing baby. Be glad for them, if they did not stretch, the skin would split. You can help to ease stretch marks by keeping the skin hydrated with moisturizer or body lotion. You will not find anything that works very well to prevent the stretch marks or to get rid of them, short of possible laser therapy after you give birth. They may be red and angry looking at first, but with time they turn into a shimmery whitish pink, that blends in fairly well.
You may notice more backaches as the pregnancy advances. This is normal. You may start to do the pregnancy waddle as you walk. Your body is feeling the pressure of the added weight that extends out from your belly and this puts pressure on your back and legs and feet. The solution is to take it easy, rest with your feet up as often as you can. You may not be able to stand for long periods of time and that is okay. Find a chair and sit down. Most people are very understanding with pregnant women, giving them plenty of room and opportunities to rest.