I know that expectant mothers have a lot on their minds as they prepare to welcome a baby, but I urge all mothers out there: please don't forget about the flu. A CDC study shows while the number of children getting vaccinated is on the rise, adults are still lacking in protection. For pregnant women, this could be a big problem.

Expectant mothers must be encouraged to get a flu shot – and if they notice flu-like symptoms, they need to seek medical help immediately.

The mortality and complications of the flu in pregnancy are enormous. And these complications are not only significant to the mother, but can also affect her baby by causing premature labor, premature delivery and even death.

There are four key issues that make the flu so problematic in pregnancy.

The first one is that pregnant women have more difficulty in fighting off infections – both viral and bacterial. The reason for this is that the immune system often is underperforming due to the hormonal changes in pregnancy, which can have effects similar to those seen in people on chronic steroid medications.

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Once a severe infection develops, the chances the virus or bacteria can overrun the mother are proportionally higher.

The second point has to do with those pregnancy hormones themselves, in particular – progesterone. One of the effects of progesterone in pregnancy is that it creates capillary engorgement and swelling of the lining of the nose and oral pharynx. This predisposes a pregnant women to contract viruses and infuse them quickly into circulation.

Next, there are significant physical changes in the breathing system of a pregnant women. There’s an upward displacement of the diaphragm, which grows over time as the belly becomes bigger – and means the total lung capacity is decreased.  What happens is the expiratory reserve volume and the residual volume of the lungs are decreased by 20 percent, so you have less air every time you take a breath.

Now, imagine having the flu and having your lungs full of mucous. With these changes, if the patient is lying down, the chest wall function makes it harder for her to breathe.

This brings me to my last point of significance. The need for oxygen is much greater in pregnant women than non-pregnant women. The reason there is an increased oxygen consumption is because you’re now breathing for two -- literally.

Ultimately, the flu can infect a pregnant woman’s lungs, not only with the flu virus, but with a secondary infection – like bacterial pneumonia, making oxygen delivery to the mother and child more problematic – and could even lead to death.

If you are pregnant, and haven’t already gotten your flu shot, you should talk to your doctor about it right away.

Dr. Manny Alvarez serves as Fox News Channel's senior managing health editor. He also serves as chairman of the department of obstetrics/gynecology and reproductive science at Hackensack University Medical Center in New Jersey. Click here for more information on Dr. Manny's work with Hackensack University Medical Center. Visit AskDrManny.com for more.