The best solution is to delay changing your job until after the pregnancy is over, because that removes the concern of coverage variations and gives you more peace of mind. Until a couple of years ago, you could have found yourself without insurance because of the preexisting condition called pregnancy, but federal regulations have gone into force recently that actually require insurers to accept pregnancy as a matter of course rather than treat it as an illness or other malady.
In 2010, the U.S. Congress passed a bill, the Affordable Care Act, which requires sweeping changes to health insurance. These changes, rather than go into effect all at once, are spread out over many years, ranging from 2012 until 2020. One of these changes requires insurance companies to accept pregnancy, and became effective in January of 2013.
This means that your pregnancy will not prevent you from getting insurance at your new insurer, but you may have to wait a period of time before you are eligible for coverage under your employer. It is not that you can be turned for pregnancy, it is a common regulation of health insurance companies to only offer enrollment during an annual application period. In the meantime, you can make use of another federal law to make sure that your medical needs are covered.
COBRA, short for the Comprehensive Omnibus Reconciliation Act, requires insurance companies to extend your coverage for up to 18 months after you are terminated, quit, or change occupations. Under the law, you are required to pay an initial processing fee, and you will have to pay the full premiums of the plan, including any portion previously paid by the employer. The idea of the law is to prevent you from surreptitiously losing your coverage just because you changed jobs, and while it may cost you a little more it will also prevent you from suffering a lapse of coverage during your pregnancy.