"She has given birth to a baby girl in Genéva in Dec/Jan and having planned to leave her with an adoptive family."
Can it really be narrowed down to this period? The Christmas special opens in the summer of 1923 and Edith has just returned from being away for 8 months. Thus the earliest she could have left is October of 1922. At the time it appeared she was at about the end of the first trimester or beginning of the second. Assuming the child was not significantly premature, and there was no indication that there were any complications, then it would seem that she was most likely to come due in the February to April time frame.