Pregnancy can be detected in first ten days. Early pregnancy is time to get used to news and readjust your lifestyle.
I guess if it is not your first pregnancy, you already have all information and, more important, intuition you need to go through. If you are pregnant first time, you may find something useful in my recollections.
Early pregnancy detection: over the counter pregnancy test are all the same. Each and every one of them registers the presence of chorionic gonadotropin (hCG, h stands for "human") in your urine with accuracy of 97%. Some are more sensitive than others, which means they detect lower concentration (listed on a package) of hCG. If you want more assurance, you could have a blood test. It is a bit more sensitive, no more accurate, and tests for hCG as well. Hormone appears in women's bloodstream (and urine) very early in pregnancy, usually by the first day of missed period, or in a week after conception. It is generally recommended to take the pregnancy test in the morning, when urine is most concentrated. Both false positive and false negative results may fall in this missing 3%. Some false positives mean early miscarriage or pharmacological interference from certain drugs. False negatives mean too low yet to detect level of hCG. Both types of mistakes can result from unaccurate application of test. Make sure to follow the instruction closely.
Early pregnancy symptoms: there are many, but none are reliable. They all can persist and cause some discomfort even after you are positive you are pregnant and need no more assurance. I superstitiously believed this is nature's way to make a woman mind her condition. Some people have morning (or not just morning) sickness. There is no known cause to it. There is no way to predict it. I was absolutely happy with first two pregnancies and suddenly sick in the beginning of the third: every pregnancy is different, just as children are. There is no cure to morning cickness. Some cautious changes in diet may help. Obviously, is you cannot stand some food, don't eat it, even if it is considered very healthy for pregnancy or used to be your favorite. If you are generally well-nourished, you will not starve you baby in first couple of month even if you loose some weight to sickness. It is very important to stay hydrated.
Some women have food cravings and some cannot stand certain foods. I read in a medical manual attributed to Galen (Greek, second century A.D.) that if a pregnant woman wants something unwholesome food, let her have it, but mixed with wholesome food. The ideas about which foods are wholesome and which are not changed countless times since then, but the advice remains sound. I had headaches during most of my second pregnancy, and my OB/GYN suggested rather secretively that I can have some coffee if I need it (which I did not, and no cause or cure was found).
Some women experience changes in their body: tender and/or swollen breasts, changes in vaginal discharge, increase or decrease in libido, both normal. Some women are constantly hungry. Overeating is never healthy. Pregnant women don't need to eat for two even in last trimester. There is certain risk that if a woman gains too much weight in first trimester, a baby can arrive with low birth weight. However counterintuitive it sounds.
Speaking about early pregnancy detection: it is generally believed that there is no safe dose of alcohol for a baby. Which means no alcohol between first unprotected sex after a period and first day of next period. I believe a drink a week will not harm anyone, but I wasn't willing to try it on my future children. Other well-known risk factors are: smoking, studied on people smoking at least two packs a day; illegal drugs, with all added risks of questionable lifestyle; certain viruses. A common cold, which is local to respiratory system, will not endanger your baby. Flu generally would not, but high fever may. Rubella is very dangerous in early pregnancy, it leads to severe brain and nervous damage in fetus. Currently in the US children receive rubella vaccine. I didn't have one, nor had rubella in childhood, and my OB/GYN was telling me to keep away from any children until I give birth, which was awkward. I was given the rubella shot right behind the door of maternity hospital, before I was released with my new daughter. Toxoplasmosis, which is a non-viral infection, is very common (some estimate 40% of all people had it) and generally harmless. If you ever had a cat, you most likely survived toxoplasmosis with no more when a sneeze. However, if you believe you were not exposed before, make sure you don't clean after cats or eat undercooked fish while you are pregnant, as toxoplasmosis can lead to birth defects. Raw fish and meat can also carry hepatitis or parasites. New drugs are not, obviously, tested on pregnant women and considered unsafe until there is enough evidence of the contrary. Many well-known drugs have well-known risks. I would say avoid unnecessary medication and choose the doctor who's judgment you trust for necessary. Check prescription carefully: doctors do make mistakes, and pharmacists do misread scrambled medical handwriting. Ask for printed prescriptions if you can. Read the small print. Your measure of precaution depends upon how paranoid you are. I was very paranoid.
Despite my paranoia, I spend whole fist trimester of my first pregnancy at home, in a new country, reading a textbook on human genetics (not a good choice for paranoid pregnant women) and drinking organic milk by quarts. As my taste changed, I found I could not tolerate conventional milk any more, and I gradually switched to mostly organic diet. Milk is recommended during pregnancy, but not mandatory. I just happened to like it. I didn't go to OB/GYN until well into second trimester, didn't have pregnancy vitamins, and had a huge baby girl in time and with no trouble at all. With my second pregnancy I was nervous about sushi I ate before I knew I was pregnant, and I went to doctor early to have a specific test, which came out negative.
A hospital will accept you in labor even if they never saw you before. There are recommended pregnancy monitoring schedules which are rather redundant for a healthy pregnancy. However, some tests may bring you peace of mind. Ultrasound, which performed in early pregnancy, usually at sixteen weeks, can tell if baby develops normally and calculate more accurate due date. It is often possible to determine baby's sex at this point (and sometimes as early as ten weeks, but doctors usually don't suggest ultrasound this early). Genetic screening is highly recommended if you and baby's father belong to same risk group for certain recessive genes diseases. These groups include people from eastern Mediterranean, Jews from Eastern Europe, and some others. You don't have to repeat this test with new pregnancies if you keep you records. Amniocentesis is a amniotic fluid sampling performed to test for chromosomal abnormalities at fourteenth to twentieth week. This test is invasive and brings certain risk of miscarriage. It is not recommended unless a woman is in a risk group or older than 35 years. Later in pregnancy you will have blood sugar tasted for diabetes.
See also: healthy food, due date.