In Mary's personal letters....

[L Verge]

First, I doubt very seriously that Mr. Lincoln was present in the bedchamber when any of his sons were born. The "birthing" process was something that no proper gentleman of the 19th century was expected to witness. Even doctors were trained to maintain eye contact with the mother while delivering (and even during any pregnancy exams - which were very rare). The exams and the birth were done more by touch than visual exam. If a baby came quickly without much warning, a husband might be forgiven for assisting his wife if he were the only person present. Some husbands were known to go fetch the midwife (more readily available than doctors) and then stay in the local tavern until it was over. His wife could then yell, scream, and curse to her heart's content during delivery.

As far as any further sexual relations after Tad's birth, if Mr. Tripp did not (likely could not) produce sources for that information, I think it is fair to Mr. and Mrs. Lincoln to say that he concocted that statement to suit his purpose. Sexual relations were just something that polite society did not discuss until well into the 20th century. Ladies may have made reference to it and female woes in the privacy of their parlor among close female friends and relatives and made slight, discreet mention in letters to such friends and relatives, but I don't think that would be the norm. IMO, and I haven't read his book (and won't), Mr. Tripp is taking buttons of innuendo and making a frock coat out of them.