Source:http://linkedlifedata.com/resource/umls/id/C0032979
MSH: The beginning third of a human PREGNANCY, from the first day of the last normal menstrual period (MENSTRUATION) through the completion of 14 weeks (98 days) of gestation.,NCI: The period of gestation that ranges from the first day of the last menstrual period in a woman up to the 12th week of her pregnancy.