Hi. To answer your question directly, No! Weight loss is not a sign of pregnancy.
In fact, during pregnancy, you are most likely to gain weight. The average weight you would gain is around 11 Kg. This weight gain again is not symmetric. In the first 3 months, you might gain about 1 Kg. The next three, about 5 Kg and another 5 Kg in the later part of the pregnancy.
You might think this weight gain is due to the growing baby inside, but this only contributes minimally to the weight gain. During pregnancy, the volume of blood in your system increases. Also, there is retention of water in your body and fat starts to get accumulated and increased. All these contribute to the increase in weight.
However, a slight weight loss is noticed in the early weeks of pregnancy. Pregnancy causes an increase in hormones in your body. This causes you to feel nauseous and may cause vomiting, especially in the morning. This is known as morning sickness. In some people, this vomiting happens in excess. They vomit soon after they eat. So, in effect if this happens, then you would be consuming very less food and this will cause weight loss. It happens only in few people and hence is not a sign of pregnancy.
If you are in doubt about the pregnancy, the most likely sign or rather an almost sure sign of pregnancy is a missed period. If you miss your dates by more than 10 days, then suspect pregnancy. You can easily confirm it by doing a urine pregnancy test. Pregnancy test cards are available in any pharmacy and is the easiest way of confirming if you are pregnant.