Short answer. Generally, yes. Rule 18 treats a party's failure, without just cause, to bring the required evidence to pre-trial as a waiver of the right to present that evidence. The rule does allow reserving evidence not yet available at pre-trial, but only by following the specific manner it prescribes.
What the law says
The failure without just cause of a party and/or counsel to bring the evidence required shall be deemed a waiver of the presentation of such evidence.
Rule 18, Section 2 — Nature and [p]urpose. Read the full provision →
The waiver rule for missing evidence
Rule 18, Section 2 answers your question directly: the failure without just cause of a party and/or counsel to bring the evidence required shall be deemed a waiver of the presentation of such evidence. Forgetting to bring evidence you were required to bring to pre-trial, without a just cause for the failure, results in losing the right to present that specific evidence later — the rule treats the omission as a waiver, not a mere delay.
What pre-trial requires you to bring in the first place
The requirement exists because pre-trial is built around exchanging and marking evidence early. Parties must mark their respective evidence if not yet marked in the judicial affidavits of their witnesses, examine and compare the adverse party's evidence against copies to be marked, and manifest for the record any stipulations on the faithfulness, genuineness, and due execution of that evidence. Skipping this step is what triggers the waiver consequence.
The "without just cause" qualifier
The waiver is tied specifically to a failure without just cause. That phrasing leaves room for a genuine, justified reason for not having the evidence on hand — the rule does not treat every missed item automatically as a deliberate abandonment of it. What counts as just cause is not spelled out in this section, so whether a particular explanation qualifies is assessed on the circumstances presented.
The narrow route for evidence not yet available
Rule 18 does provide a specific way to preserve evidence you genuinely could not bring: parties may reserve evidence not available at the pre-trial, but only in the following manner — for testimonial evidence, by giving the name or position and the nature of the proposed witness's testimony; for documentary or object evidence, by giving a particular description of it. The rule is strict about the form: no reservation is allowed if it is not made this specific way. A general statement that more evidence will be produced later, without the specific description the rule demands, does not satisfy this requirement and leaves that evidence exposed to the waiver rule described above.