Trial starts today in the case of a Lawrence man charged with killing his 5-month-old daughter – a case that likely will center on conflicting medical opinions and the word of the baby’s mother.

The defendant, 27-year-old Jay D. Decker, was in District Court all day Monday for jury selection, which ended late in the afternoon.

Prosecutors allege Decker repeatedly abused Risha J. Lafferty in the weeks leading up to her Oct. 14 death. A coroner testified he found 35 bruises or other external injuries on Risha – some that were several days old – and that she had a fractured skull, bruises under her scalp and signs of shaking.

At the time, Decker was watching the child during the day at Edgewood Homes, 1600 Haskell Ave., while her mother, Brandi Mae Hendrickson, was at work.

Hendrickson was charged with felony child endangerment because she knew of some of the injuries but didn’t report them. But prosecutors allowed her to plead to a misdemeanor, and she is expected to be the key witness against Decker.

Defense attorney Mark Manna questioned Hendrickson’s credibility during a preliminary hearing earlier this year. For example, he pointed out that she initially told police she’d gone to bed at 11 p.m. the night before Risha’s death but later testified she went to bed at 10 p.m.

The defense has hired its own medical expert.