22 weeks after conception (5 ½ months)
(24 weeks after the last menstrual period)
The fetus is about 8 ¼ inches (8.25 inches) from crown to rump (head to butt) which is about the length of a cucumber and weighs about 1 ¼ pounds. The lungs may just be beginning to mature. A fetus born at 22 weeks will have difficulty surviving.
The average cucumber is 8 inches in length. 
Click here for a list of clinics offering free pre-natal ultrasounds