The Precursors, the masters of the Kaiju, never officially reveal themselves or their plan for world domination. Sending a pregnant Kaiju through the rift is a win for the Precursors because if a species isn't strong enough to stop the Kaiju, then the Kaiju gives birth and destroys the target planet just that much faster.
What we know
The only information we have regarding the thinking processes of the Precursors comes from two scientists who, using the mind-linking software commonly called The Drift, used to pilot the Jaegers, link their minds to a dying Kaiju and discover the intent of the Precursors in the minds of the Kaiju.
With that said, the reason for sending a pregnant Kaiju into combat cannot be definitively established, we are forced to study the behavior of the Precursors and their proxies, the Kaiju by their actions.
The Kaiju began their assault on the Earth five years prior and they decreased their time between attacks, slowly and steadily. Using a mathematical model created by Hermann Gottlieb it was determined the creatures were appearing with a mathematical precision indicating intelligence behind their arrival.
But it wasn't until the scientist Newton Geiszler performed a drift with the Kaiju mind which indicated true intellects sending the Kaiju through the dimensional rift between the Earth and the Precursor dimension.
Geizler determined the Kaiju which attacked the Earth were all genetic clones of previous Kaiju, exact cellular copies. This genetic capacity indicated a high level of technological sophistication on the part of the Precursors. It was implied the Kaiju may be little more to the Precursors than biological weapons used to clear planets of infestations. (Any lifeforms which lives on a planet the Precursors want to own are considered vermin.)
Pregnancy
If we take the information that the Kaiju CAN be pregnant, it could means the species (or technology, depending on your perspective) is possibly parthenogenetic and/or self-fertilizing. If that is the case, becoming pregnant may simply be part of the natural cycle of the Kaiju, assuming they weren't being killed by the Jaegers. Note the Kaiju that was pregnant wasn't immediately killed upon arrival. It might be that ALL Kaiju were capable of this process but had not survived long enough to activate the ability. From the perspective of an invader, having a bioweapon capable of breeding itself means a successful unit which gets to breed is more liable to clear the planet sooner and more effectively.
or
The alternative consideration that the pregnant Kaiju was just an opportunity to send two for the price of one. If the first Kaiju could settle down and establish itself, then a pregnancy would allow it to continue to wreck further havoc using a fast growing offspring.
