Accurate ultrasound dating conception
The level of the amniotic fluid also is measured and used in the calculations.
The timing of the ultrasound affects the accuracy of determining the gestational age. Later in the pregnancy, babies grow at much different rates, and babies at the same gestational age are more likely to vary in the measurements taken in the ultrasound.
The ultrasound machine and software allows the technician to see the baby's development and measure the baby.
In the first trimester, the ultrasound measures the fetus from head to the pelvis, since the baby is so small.
It's wonderful to see a little blob with a heartbeat that will become your baby. If you are unsure of the first day of your last period you will be offered a dating ultrasound when you are between 10 and 14 weeks pregnant.
But ultrasounds can also lift the lid on a few worries you might be having. The main reason for the ultrasound is to work out how many weeks pregnant you are, and to estimate your due date.
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.
Using the date of the last period gives a general idea of that information, but abnormalities in the cycle or inaccuracies in the date affect the calculations.
Ultrasounds commonly are used to determine the age of a fetus.