Fetal age is best expressed in weeks. Most uncertainty comes from the use of both calendar months (28-31 days) and lunar months (28 days). Most use calendar months.
GESTATIONAL PERIOD is divided into 3 trimesters or parts of 3 calendar months each
By the end of the first trimester, all major systems are developed, and CR length is about as wide as one's palm.
At the end of the second trimester (26 weeks according to LMP but only 24 weeks related to time of fertilization), the fetus is too immature to live independently and is about the size of a handspan.