It is a rather complicated question whose answer involves several factors。 First let us assume that you are talking about a case similar to (1)。
(1) "John was born in 1988," says Mary。
When you turn it into reporting speech, you face the choice whether you use present or past tense with the verb in the matrix。
If you decide on the present, you do not need to shift the tense in the embedded to past perfect。
If you use the past tense with the matrix verb, you, theoretically speaking, need to alter the tense of the embedded verb, too。
But in the case at hand you cannot do that, otherwise you will create a rather awkward sentence like "Mary said thatJohn had been born in 1988"。
The reason for this is that when you give consideration to the tenses in reporting speech, you need to make a three-way distinctioon among the reference time, the speech time and the event time。
Suppose that you are reporting Mary's speech, the reference time is set at the present, then the event of Mary's saying something may be either present or past, that is why I have told you that you face a choice eearlier。
If you choose the past, the event of John's being born is one before Mary's speech, viz。
, past in the past, and it is theoretically predicted that you should use the past perfect with the verb in the embedded clause。
That is why I have just said you need to alter the tense of the embedded。 But as there is an adverbial of time in the embedded clause which is anchored out of your referential system, so there is another possibility that you determine the tense of the embedded。
Compare the two sentences in (2)
(2) a。 Mary said that John studied in China in 1988
b。
Mary said that John had studied in China three year before。 。