Praise be to Allaah.
There is no doubt that a woman's pregnancy and giving birth is fulfilling an aim of shareeah that is dear to Allah, which is increasing the progeny of the Muslims who proclaim the Oneness of Allah, the followers of the Prophet (blessings and peace of Allah be upon him). This is the most important thing on which the woman should focus her intention during her pregnancy.
It was narrated that Maqil ibn Yasaar said: A man came to the Prophet (blessings and peace of Allah be upon him) and said: I have found a woman who is of good lineage and beautiful, but she cannot have children; should I marry her?
He said: No.
Then he came to him a second time and he told him not to marry her.
Then he came to him a third time and he said: Marry the one who is loving and prolific (could bear many children), for I will be proud of your large numbers before the nations.
Narrated by Abu Dawood, 2050; al-Nasaa'i, 3227; classed as saheeh by al-Albaani.
Because of this aim to which we have referred, if pregnancy is difficult for the woman and she bears it with patience, there are many benefits which will come to the mother through it, including the following:
Psychological preparation for the task of raising the child which is the most important and most complicated of all tasks, in which they (the parents) raise the child to have a good attitude and become religiously committed for the sake of Allah, and they hope that Allah will decree reward for them because of the actions of their righteous son, so that he will become an ongoing charity for them after they die and thus they may attain a great deal of reward which no one knows except Allah.
The difficulties encountered by the pregnant woman, including pain and sickness, and health, psychological and physical problems in many cases, will all bring great reward that will be recorded for the pregnant woman, in sha Allah. Allah will reward the Muslim for everything that befalls him in this world. Even if a thorn pricks him, Allah will expiate thereby some of his sins, and the pains of labour and pregnancy are far greater than that.
Even if we assume that this woman dies in childbirth, she will die as a martyr. This is indicative of the virtue of her condition. The Prophet (blessings and peace of Allah be upon him) said: The woman who dies in pregnancy or childbirth is a martyr. Narrated by Abu Dawood, 3111; classed as saheeh by al-Nawawi in Sharh Muslim, 13/62. And he said: the one who dies in childbirth, i.e., she dies with something (the child) inside, not separated from her.
As for the acts of worship that the pregnant woman can do, they are all the acts of worship that the Muslim does by day and by night, such as praying, fasting (so long as there is no fear of harm), giving charity, reading Qur'aan, regularly reciting the adhkaar that are prescribed in shareeah, treating people kindly, visiting relatives, taking stock of oneself, and striving to attain the best attitudes, actions and words.
Perhaps one of the things that the woman should focus on during this period is learning about sound methods of raising children, reading books on this topic or listening to useful lectures by scholars on it, whether that has to do with moral upbringing, health, psychology, pedagogy, and so on, in preparation for the great mission with which Allah has entrusted the parents, which is the trust of raising and caring for the child, so that the parents may embark upon it with knowledge and insight and achieve the best results, and attain the pleasure of Allah in this world and in the Hereafter.
With regard to specific rituals or adhkaar specifically for the woman during pregnancy, we did not know of any such thing in Islam.
Finally we should point out that there are some hadeeths which indicate that the wife's pregnancy will bring a reward like that of one who fasts and prays at night for the sake of Allah, and that there are many other rewards for giving birth, breastfeeding and weaning the child, but they are all false and fabricated hadeeths, so it is not permissible to narrate them or speak of them except by way of warning people about them. We have already mentioned some of them on our website in the answer to question no. 121557
And Allah knows best.