CDAR2 C-CDA Supplemental Templates Pregnancy, Release 1 - US Realm has Unknown Pregnancy Documented with nullflavor for observation and value.
HL7 CDA® R2 IG: C-CDA Templates for Clinical Notes STU Companion Guide, Release 4.1 - US Realm has Unknown with nullflavor for observation only and coded value of pregnant for value.
CDAR2 C-CDA Supplemental Templates Pregnancy, Release 1 - US Realm has Unknown Pregnancy Documented with nullflavor for observation and value. HL7 CDA® R2 IG: C-CDA Templates for Clinical Notes STU Companion Guide, Release 4.1 - US Realm has Unknown with nullflavor for observation only and coded value of pregnant for value.
Likewise the example here - https://github.com/HL7/C-CDA-Examples/blob/master/Social%20History/Unknown%20if%20Pregnant/Unknown%20if%20Pregnant.xml also has nullflavor for obs and coded value of pregnant for value.