Antenatal corticosteroids for early preterm birth: implementation strategy lessons from the WHO ACTION-I trial

Table 1 Using ultrasound and LMP to determine ACTION trial estimate of GA [6]

	Timing of earliest ultrasound to estimate GA (by LMP)
	First trimester		Second trimester			Third trimester
	Up to 8 weeks 6 days	9 weeks 0 days to 13 weeks 6 days	14 weeks 0 days to 15 weeks 6 days	16 weeks 0 days to 21 weeks 6 days (by LMP)	22 weeks 0 days to 27 weeks 6 days	28 weeks 0 days and beyond
Method of measurement on ultrasound	CRL		BPD, HC, AC, FL^a,b
LMP is certain	If difference is ± 5 days, use LMP If difference > 5 days, use U/S	If difference is ± 7 days, use LMP If difference > 7 days, use U/S		If difference is ± 10 days, use LMP If difference > 10 days, use U/S	If difference is ± 14 days, use LMP If difference > 14 days, use U/S	If difference is ± 21 days, use LMP If difference > 21 days, use U/S^c
LMP is uncertain or unknown	Use U/S based on careful clinical judgment

LMP last menstrual period, CRL crown–rump length, BPD biparietal diameter, HC head circumference, AC abdominal circumference, FL femur length, U/S ultrasound
^aTranscerebellar diameter (TCD) can also be optionally used, from 14 weeks 0 days to 27 weeks 6 days
^bIf an earlier dating U/S is available, and is consistent with LMP, use the GA from the earlier scan
^cIf no earlier scan, check TCD, distal femoral, proximal tibial and proximal humeral epiphyses in addition to biometry

ISSN: 1478-4505