A fetus is very clever, even at an early age! Normally, your body would see something like a pregnancy as a foreign object. But your baby has other plans.
Your immune system is suppressed and prevented from attacking and rejecting the foetus, thanks to hormones and antibodies the foetus produces
The antibodies also take part in the growth and development of the placenta. As a result of these hormonal changes to your immune system, you are more susceptible to colds and flu.
Nasal stuffiness is also very common in pregnancy due to the hormonal effects on the nasal passages