Unless you have a superhuman extra sense, you probably won't know for sure that you are definitely pregnant in the earliest weeks of pregnancy. There are signs of pregnancy that might occur but not all manifest themselves and it really varies per person.
Changes in your body
Your uterus (womb) will start to enlarge but will not be noticeable to you at this point. However, your hormonal levels change immediately (you start producing a hormone called 'human chorionic gonadotropin' or hCG, which is what a pregnancy test kit looks for in a urine sample) and this can affect your breasts, make you feel particularly tired or bring on morning sickness.
You may feel emotionally unbalanced too, with extremes coming on unexpectedly. This is a completely natural aspect of the hormonal surge and not a message from your body that something is 'not right' or that you should be worried about being pregnant.
What is happening in your womb?
Within the first six weeks, all the building blocks are put in place. The brain, heart, spine and skeleton are being formed in this period even though the body is still barely perceptible - less than a centimetre in size.
For more about fetal development, don't forget to check out the fetal development articles and sign up for your fetal development newsletters which will tally with your due date.
What you should do
Whether or not you have tried using a pregnancy test kit at home, go to see your GP. A urine sample will be taken and you can be formally confirmed pregnant. This means you will then be referred to a hospital and a midwife team for ante-natal care, and appointments can be made for you to have a dating scan (which takes place around 10 to 13 weeks). You will also be eligible for free prescriptions from now, but need to register for this via your GP.
If you have been trying for a baby, you should treat each month as if you are pregnant until your period appears. Avoid alcohol, smoking and taking drugs during this time, as vital organs are being developed. Cutting down on caffeine can help too.
Not everyone is consciously trying for a baby and it is best not to worry if you did not realise you were pregnant whilst 'partying' - what's done is done. Plenty of women will have been doing all kinds of things to their bodies, yet their babies are perfectly fine. However, once you do know, it's wise to treat your body with a bit more respect as a serious precaution.
Don't worry too much right now about birth choices and so on, instead just concentrate on getting yourself into good habits to prepare for pregnancy lifestyle changes and allow yourself time to get on top of a good pregnancy diet.
And don't forget... you can always visit the ThinkBaby Forums to ask questions and talk to other women going through what you are going through. This is a time when there are lots of unknowns but you don't neccessarily want everyone around you to know you are or might be pregnant, so coming to ThinkBaby where you can be anonymous, can really help!