Mom's Heart is the final boss of The Womb II. In the beginning of the battle it is always accompanied by two Eyes. If you listen closely to her theme song, you can hear heart beats.
Strategies
Mom's heart spawns two to four monsters and then disappears. After killing those monsters, the heart will reappear in the middle of the room for 3-4 seconds, spawn more monsters, then disappear again. If you manage to kill the monsters within two seconds from their spawning (before the Heart disappears), Mom's Heart will stay on screen and spawn a next pack.
Provided you have bombs, placing a bomb just as the heart is about to drop into vision will not only damage the heart, but may also destroy any monsters the heart spawns; this can afford you essentially double the attack time against the heart.
When Mom's Heart drops down to 33% health, it will stop hiding but continuously spawn monsters and start firing bombs similar to those fired by Sloth and other monsters. The explosions will leave glowing puddles of blood, similar to the ones left by the brains, that do damage on contact and last for several seconds. The monsters it spawns in this phase are usually weaker, consisting largely of various small flies.
Alternatively, using 'The Bible' will act as an instant kill, similar to using it on Mom. This is not to be confused with the similar looking 'Book of Revelations' item which has no effect on Mom or Mom's Heart.
Also items such as the My Little Unicorn and the Gamekid do NOT work on her. She will not take any damage but may be useful to fight the monsters she spawns.
After you defeat Mom's Heart 9 times in the game it becomes a giant Isaac in his mother's womb, confirmed by the game creator Edmund McMillen. (See It Lives)
Bugs
The bug, and the chest that appears after you defeat Mom's Heart.
When defeating Mom's Heart using the Bible, the health remaining bar will not disappear, but the game will go on with the ending as usual and the chest will appear (seems to have only appeared since WotL DLC)
The devil room after Mom's Heart sometimes will not show up at all and the player is forced to continue and opening the golden chest. (only appeared since WotL DLC)
The firing bombs, similar to those fired by Sloth, can get you out of the map and then you will be unable to finish the game (because you are stuck in an invisible wall).It Lives.
The Duke of Flies • 
Gemini • 
Monstro • 
Larry Jr. • 
Fistula • 
Gurdy • 
Peep • 
Chub • 
Loki • 
Scolex • 
Blastocyst • 
The Fallen
Monstro II
Steven • 
C.H.A.D. • 
Gish
Famine • 
Pestilence • 
War • 
Death • 
Headless Horseman
Mom • 
Mom's Heart • 
It Lives • 
Satan
