You know something is "off" with you or your partner but you may not be able to put your finger on what it is. This "off" can show up as putting on the brakes in some way (i.e., withdrawn, shut down, disconnected) or having a strong, emotional reaction that does not seem warranted. Either severe reaction can be experienced as a surprise at first for the other partner. This can occur in public or privately whether in the bedroom or during a moment of emotional closeness.
In general, trauma can affect a couple based on when it occurred to one or both of them:
trauma during the current relationship
trauma in adulthood over the age of 25 before the current relationship (whether during a past relationship or not)
trauma during one or both partners' lives up to the age of 25 (while the brain is still developing)
If not addressed, trauma from any of these time periods can affect the relationship, especially if the person who experienced the trauma had isolated themselves after it occurred. For the purpose of this writing, I will cover the first two briefly since the last one can affect a couple the most.
This can be from a single trauma event that occurred to either or both partners. Usually the trauma event (i.e., car accident, witnessing or surviving a violent event) has been minimized or it can be domestic violence. In most cases, the event(s) can be identified.
Before The Current Relationship
Trauma from a past relationship can interfere with the current relationship, especially sexual trauma if it has not been processed. Not only can unprocessed trauma affect a couple's sex life, it can also cause havoc to the relationship in other ways. The person in the relationship who has experienced trauma may show defensiveness that may be interpreted as sabotaging the relationship. As tempting of a conclusion as this is, don't buy into it.
This so-called sabotage is actually signs of self-preservation that is prioritized over what's good for the relationship. Their defensiveness is actually protection because the brain is wired to prevent past trauma from being repeated at all costs, whether in a relationship or not. This can be manifested in many ways: anticipatory anxiety, suspiciousness, catastrophising, hostility, anger, dissociation, numbness, or even memory loss.
Before Fully-Developed Adulthood
If trauma occurred in one's life while the brain is still developing (birth to age 25) a person's defenses/protection can be more ingrained, especially if the trauma occurred over a long period of time rather than a single event. Oftentimes, this type of trauma is prevalent within one's family either as a violation of boundaries (i.e., physical, emotional or sexual) or neglect (i.e., abandonment, disregard or intentional put-downs).
Chronic trauma early in one's life can shape one's personality and can be harder to pinpoint but can be manifested in relations to others: social anxiety, overtly apologetic, avoidant, unemotional, or self-centered to name a few.
If a person experienced trauma earlier in life, most likely they learned their emotional needs are not going to be fulfilled by another person even their partner. Instead they develop a self-sufficient support system. If this person experiences conflict in a relationship, oftentimes they will distance themselves from their partner, shut down, or sometimes do something solitary such as play video games or spend time on their phone as a way of escaping. A huge risk for this person is to step out of their self-sufficiency by asking their partner for the emotional support they never got in their life. It's difficult because they anticipate they will be disappointed (again), reinforcing the belief that they are alone.
In couples therapy, this self-sufficient support system is acknowledged in the partner who withdraws. Time is spent in therapy making it safe for this partner to begin the process of transitioning their self-sufficiency to a healthy, emotional interdependence with their partner.