However, if pregnant women presents for the first time in 2nd trimester of pregnancy, then the parameters used in ultrasonography to calculate gestational age of foetus is femur length (FL), Head circumference (HC), Biparietal...
Ultrasound is a painless, non-invasive and rapid technique to confirm pregnancy, evaluate gestational age and EDD (expected date of delivery). The presence of gestational sac on USG scans is considered as the 1st definitive sign of pregnancy and the presence of yolk sac within the gestational sac is considered as the 1st reliable sign of intrauterine pregnancy.
Best time to calculate gestational age of baby is the 1st trimester of pregnancy (9-12 weeks) by an ultrasound scan (using the parameter CRL). Gestational age is calculated using CRL (Crown-rump length) in the 1st trimester of pregnancy as following-
1)To calculate days of pregnancy, formula used is CRL (in mm) + 42.
2)To calculate gestational age in weeks, formula used is CRL (in cm) + 6.5.
However, if pregnant women presents for the first time in 2nd trimester of pregnancy, then the parameters used in ultrasonography to calculate gestational age of foetus is femur length (FL), Head circumference (HC), Biparietal diameter (BPD) or Abdominal circumference (AC).
And if pregnant women presents for the 1st time in 3rd trimester of pregnancy, then parameters used for the estimation of gestational age includes femoral epiphyseal ossification centers, proximal tibial ossification centers or proximal humeral ossification centers.
