nothing
YES
Yes you can spot during the first week of you pregnancy. The egg is being planted into the uterus.
Usually after the second.
Yes,This could be painful inplantation of the egg.
A good resource for what one can expect during the first trimester of pregnancy is the book titled What to Expect When You're Expecting. It has a week by week guide as to the progress of ones baby. It answers many questions one may have during ones pregnancy.
No. If you are one week pregnant, technically you will not have missed your period yet. Pregnancy is counted from the first day of your last period. So during week one, you are on your period. The pregnancy will usually be detectable around week 3. That is when the egg meets the sperm and implantation occurs. Once the ovum (egg/sperm) has implanted, your body will start creating the hormone HCG (that's what the pregnancy tests are looking for).
This can be confusing! The first week of pregnancy is actually considered to be the day of your last menstrual period, before you are technically even pregnant! You are actually two weeks pregnant when conception occurs, and generally four weeks pregnant before you can get a positive pregnancy test!
An embryo forms first during pregnancy, after fertilization of the egg by sperm. It develops into a fetus around the end of the eighth week of pregnancy.
I am not sure what you mean by cancel out a pregnancy. The only way to stop a pregnancy once it occurs is abortion. Sometimes you can induce a period before pregnancy occurs to prevent it. The pills have instructions that say exactly how many you can miss.
Babycenter's website has a very detailed information chart on how a fetus develops week-by-week during a pregnancy. The site also has an information chart on how twins develop during pregnancy.
No it is far to early to say anything about pregnancy in the first week.