This type of question falls under what moral theologians call ‘cooperation with evil’.
This site has some good resources for health care professionals on Catholic ethics:
ascensionhealth.org/index.php?option=com_content&view=article&id=32&Itemid=170
and here is the section on cooperation with evil
ascensionhealth.org/index.php?option=com_content&view=article&id=82&Itemid=171
Performing work in a lab related to tests for certain birth defects would fall under mediate material cooperation.
"Mediate material cooperation occurs when the cooperator participates in circumstances that are not essential to the commission of an action, such that the action could occur even without this cooperation. Mediate material cooperation in an immoral act might be justifiable under three basic conditions:
Code:
* If there is a proportionately serious reason for the cooperation (i.e., for the sake of protecting an important good or for avoiding a worse harm); the graver the evil the more serious a reason required for the cooperation;
* The importance of the reason for cooperation must be proportionate to the causal proximity of the cooperator’s action to the action of the principal agent (the distinction between proximate and remote);
* The danger of scandal (i.e., leading others into doing evil, leading others into error, or spreading confusion) must be avoided."
I don’t know enough about your specific situation to judge if it is moral. But speaking in general, the lab tests that you refer to do not necessarily mean that an abortion will be performed. Some parents wish to know in advance if birth defects are present, so as to prepare for that situation in the born child. So your cooperation is neither formal, nor immediate; the act that you are doing is distant from the sin of abortion, and is not necessary for that sin to occur, and so it is mediate material cooperation.