There is a very scientific reason for this as a very close relation exists between pregnancy and eclipse.
The earth, as we know consists of 75% water and only 25% land mass. Likewise a human body too is made up of 65% liquids and 35% solid mass. As per the law of nature, liquids are more easily influenced and affected by the movements of the Sun and Moon. This can be observed in the tidal waves that occur in the oceans especially during New moon and Full moon.
A human brain consists mainly of liquid content and hence easily reacts to the gravitational force of the moon. Due to this, moon is referred to as the Lord of the mind. Many mental disorders or disturbances have been noted on New moon and Full moon days. This effect can be easily viewed in case of weak-minded persons.
During pregnancy, a fetus is still in the formative stage and as such there is not much difference as yet between its mind and physical body as well as between the water and solid mass content in it. It is in such an extremely weak and vulnerable stage. Hence it is easily influenced by the Sun and the Moon rays.
It has been scientifically proven that at the time of an eclipse, the rays emanating from the Sun or Moon are charged with ultraviolet (UV) or infrared radiations. Due the effect of these radiations, the fetus will be under stress. Any additional stress on it, which could be caused by the mother’s movements, could lead to physical deformity in the fetus. In case the mother is mentally stressed out at that time, it could lead to mental disorder in the baby. To avoid these, it is advisable for a pregnant woman to lie down peacefully during an eclipse.