If you have been planning a baby, you are most likely to do a urine examination at home which will tell you if you are pregnant or not. Sometimes, if you are only in the fourth week, the test may not be able to give you a very accurate result. Otherwise, the indication might only be very light so look for it carefully.
You may not feel any physical changes in your body. You may experience a peculiar taste in the mouth which is very metallic. The only way you will be able to tell very clearly that you are pregnant is when the gynaecologist does an ultrasound or an internal examination. The egg has already gone into the uterus by now, and is slowly forming into a baby.
In your ultrasound it will look like a little ball. This is actually called an embryo. This is slowly going to form into your baby and the outer covering will start to become the placenta. In your ultrasound, your doctor will be able to show you what is called a yolk sac. This is what is helping your baby grow initially with the nutrients required, till the placenta and also the umbilical cord are formed. Eventually, it is the placenta which is going to be nourishing and feeding the baby in your womb.
Once you have discovered that you are pregnant, there is not going to be any weight gain in these four weeks. You will be able to wear clothes you have been wearing normally quite comfortably. However, if you do know you are pregnant, do not wear very tight clothes. You may begin to experience nausea or tiredness but there will be no physical difference which is noticeable by you or by others around you.
This is only the beginning. You have a long way to go. Although you will be very excited and looking forward to your pregnancy and the baby, you must take it slow. Take as much rest as you can, look at your lifestyle and make the changes that you need to. You must make an effort to be happy.