If you have an irregular cycle, or if you've recently been taking the contraceptive pill, working out the date from your LMP isn't a very accurate method.Ultrasounds are more accurate than using the LMP and, if the ultrasound date differs from your period dates, the ultrasound date will be used.The LMP is used because most women know when their last period was, but may not know when they actually conceived.If you do know the conception date, this is classed as day 14 and not day one.
This is where our wonderful ultrasound machines can make a crucial difference.
The ultrasound date is usually later than the period date, so accurate dating has been found to reduce the number of women having labour induced because their babies are overdue The ultrasound can check that your baby has a heartbeat and is developing normally.
Your baby's head, limbs, hands and feet, and some organs can be seen, and - though looking for abnormalities isn't the purpose of the ultrasound - some major problems can be seen at this early stage.
In fact, the only time that this doesn't apply is when the date of conception is known without doubt, as when an infertility patient conceives with timed intercourse or artificial insemination.
The first trimester is defined as the first 13 weeks of pregnancy following the last normal menstrual period (some authors refer to early pregnancy as 0-10 weeks).