If you're newly pregnant, you are probably 6 weeks gone already, so let's have a retrospective look at what has been happening during the early weeks of your first trimester.
The first fortnight
The duration of your last menstrual period (LMP) actually counted as week 1 of your pregnancy, as your due date will have been calculated by counting forward 40 weeks from the day you last came on. So that's the first week accounted for. Around a week later – about 14 days after you started your period – you would have conceived your baby (and given that info, maybe you can pinpoint the exact occasion). All of which means that by the end of your first month of pregnancy your baby will have done two weeks' growing.
Due date ready reckoner
You can work out your due date (accurate to within a few days) in your head. Find the date of the first day of your last period, then add on exactly a year. Go back three months and add on one week: and there you have it, your estimated due date (EDD).
As soon as your period ended, the lining of your uterus (called the endometrium) started building up again in readiness to accept a fertilised egg. Eggs were maturing in your ovaries, ready for one to launch itself into one or other of your Fallopian tubes. Once off and running, your egg will have met with a healthy sperm and become fertilised. From there on, it will have taken between 7-10 days to reach your newly refurbished uterus.
On the outside
At this stage there will be no discernible signs of pregnancy – although you might have felt some twinges or niggling pains during the time you ovulated. Some women have what appears to be a lighter period than usual, so may have no reason to suspect pregnancy.
Pregnancy weeks 3-6 weeks
You have probably discovered you are pregnant during these weeks. Home pregnancy tests are so accurate these days that most can tell you whether or not you're pregnant from the first day of your missed period. Make an appointment with your GP who will refer you to your local maternity unit. They will prescribe a supplement of Folic acid (0.4 mg) per day, which reduces the risk of having a baby born with a spinal cord defect such as spina bifida and of premature labour.
The first trimester is a busy period for your growing baby (at this stage an 'embryo' until week 8 when the term 'fetus', meaning 'young one', is adopted). The inner part of the embryo has begun growing and the outer part has started making up two membranes known as the 'amnion' (which will become your amniotic sac) and the 'chorion' (which will become the placenta). By week 6 the spine begins to form, along with a basic umbilical cord and circulation system. The embryo is about the size and shape of a baked bean.
On the outside
You might have noticed some 'spotting' (small amounts of blood) in your underwear. This sometimes happens a few days after the embryo beds down into the uterus and takes hold. You might have noticed feeling knackered for no good reason, sore or tingly boobs, going off foods you've always liked before, feeling generally a bit the worse for wear and dashing off to the loo for a wee more often than usual.