A couple are not required to check for blood after relations, but nevertheless may find it. Blood found under these circumstances raises questions about a woman’s niddah status and, in some cases, about whether uterine bleeding has been triggered by relations.
Niddah Status
Dam Makkah
Only uterine bleeding makes a woman niddah. Bleeding from outside the uterus (e.g., from the external cervix, the vaginal wall, or the labia) does not make a woman niddah.
Thus, if a woman has a known medical condition that can lead to bleeding at relations (e.g., vaginal atrophy or ectropion) or a non-uterine source of bleeding is identified afterwards, then the bleeding can usually be attributed to that condition or source. The halachic term for such bleeding is dam makkah, blood from a wound, which does not make a woman niddah.
We follow the position that bleeding from known trauma or a wound inside the uterus is also considered dam makkah, and does not make a woman niddah.
Similarly, if the husband has a cut or known source of bleeding in the genital area, blood can be attributed to that and does not make a woman niddah.
When there is bleeding during or following relations, it is generally advisable to take steps to determine if it might be dam makkah. Please see here for a detailed discussion of dam makkah, and how to check for it.
Stains or Possible Hargashah
The usual leniencies of stains apply when a stain is not found immediately after relations. Thus, a stain found on a colored sheet or towel, or on a disposable tissue, would not make a woman niddah if she finds it after some time has passed since relations.
However, the usual leniencies of stains do not apply when discharge is found immediately after relations. There is a halachic concern that the sensation of marital relations could mask a hargashah, a halachically defined sensation of menses. Discharge of a niddah color found immediately following a hargashah is generally evaluated stringently.
There is dispute among halachic decisors as to what is considered “immediately” following relations. Opinions range from a few seconds to a few minutes. We follow the opinion that fifteen seconds suffices. A question should be asked if less time passes.
On the Husband’s Body
If discharge of a niddah color is found on something that was inserted into the vagina (e.g., a bedikah cloth), it is evaluated stringently even if some time has passed. Thus, blood found on the husband’s penis after relations, or on a cloth or tissue that he used specifically to wipe his penis, would generally be evaluated stringently, even if found more than a few minutes after relations.
Ro'ah Machamat Tashmish
Halachic sources discuss the very rare possibility of a woman for whom marital relations consistently trigger niddah bleeding, known as ro’ah machamat tashmish. A couple who were to establish halachically that relations consistently lead to niddah bleeding would not be able to remain married.
In practice, a couple should consult a halachic authority as soon as possible if they believe that the wife became niddah from bleeding during or after relations, in order to ensure that they take the appropriate steps.
Recommended Precautions for Relations
A couple can often take halachic precautions to avoid niddah right after relations, as well as the issue of ro’ah machamat tashmish. These measures are designed to maximize the chances that blood found after relations will be halachically considered a stain that does not make a woman niddah. They include using dark bedding, cleaning afterwards with dark towels or with disposable products, and being careful not to look for blood.
These are recommendations and not obligatory. Please see here for a detailed list of precautions.