1. Missing your menstrual period (amenorrhea)
This is usually the first, and one of the most reliable signs that you are pregnant. It is, however, not a guarantee -- stress, a poor diet, weight loss or gain, strenuous exercise, the pill/hormonal birth control, breastfeeding and several other factors can also delay/inhibit your period.
2. Light period
Instead of missing a period, you may also experience what seems like a very light or very short period and still, in fact, be pregnant. This is thought to be implantation bleeding/spotting, caused by the fertilized egg implanting itself into the uterine lining.
3. Temperature Rise
If you have been recording your basal body temperatures (BBT), a pattern of 18 or more temperatures above the coverline generally indicates pregnancy, according to Toni Weschler's book, Taking Charge of Your Fertility.
4. Nausea, vomiting and food aversions
Often called morning sickness, nausea and vomiting are common in early pregnancy. Food aversions are also typical, particularly toward fatty and/or strong-smelling foods (onions, garlic, eggs, meat, fast food). None of these symptoms are proof positive of pregnancy, as they can be signs of an illness, food poisoning or an indicator stress, among other things.
5. Frequent urination
The need for many trips to the bathroom is also sometimes an indicator of pregnancy, and generally kicks in about 8-10 weeks after your LMP, but sometimes sooner. A bladder/urinary tract infection or diabetes, drinking a lot of fluids and use of diuretics can also cause frequent urination.
6. New pelvic sensations
Some women feel an aching or heaviness in the pelvis, which could be caused by the increased blood flow and the expanding uterus.
7. Tender, swollen or tingly breasts
Some women experience breast changes before menstruation or when they're taking contraceptives, but this also may happen during early pregnancy. The tenderness may grow more pronounced as the days pass.
8. Darkening of areola
The area around the nipples may darken due to pregnancy or hormonal changes.
Although you might be tired for any number of reasons, during early pregnancy, your body is working hard nurturing this new life and may cause you to feel exhausted. Other causes of fatigue include stress and depression, illness, and simply not getting enough sleep.
10. Food cravings
Some women experience food cravings, for everything to the nutritious to the indulgent to the bizarre. Nutritional deficiencies, PMS and stress can also trigger such cravings.
More things to keep in mind
All of the above symptoms, together with positive pregnancy tests, are only presumptive symptoms of pregnancy -- they don't guarantee anything. The only way you can be 100% sure that you are expecting is to hear or see (via ultrasound) the fetal heartbeat.
That said, if you think you're pregnant, take care of yourself as if you were pregnant. Be sure to eat well, cut out alcohol and cigarettes, and nurture yourself.
If you can't sleep at night because you're wondering if these new sensations you're feeling mean you're pregnant, and you're period's due and hasn't arrived, take a pregnancy test to give yourself some peace of mind. Sometimes, particularly if you strongly want to be pregnant or really do not want to be pregnant, your mind and body can manifest some symptoms.