If you are having the NIPT or NIPS or cell free fetal DNA screening for Down syndrome, the fetal gender can be determined at the same time. Otherwise, the best time to find out is at the 19 week anomaly scan.
Occasionally it comes up with a false positive for a sex chromosome abnormality, so any suspicious results need to be followed up with a chorionic villus sampling or an amniocentesis to be believed.
The length of your pregnancy is calculated from the first day of your last menstrual period (LMP), even though you don't actually conceive until 14 days after your LMP, and later than this if your cycle is longer than 28 days.
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.
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 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.
The accuracy of ultrasound at this gestation is over 99% as well.
Having an ultrasound in the first trimester of pregnancy can be a thrilling experience. Bear in mind that you may need a second ultrasound before you know for certain if all is well.