Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. We do not know the reasons of the miscarriages you are asking about. Also there is no specific Du'a to avoid miscarriage. However, we would like to inform you of some good acts that may help one achieve his goals. 1.Being patient and asking Allah a good reward for any calamity that inflicts a person. Imam Muslim narrated from Umm Salamah that the Prophet (Sallallahu Alaihi wa Sallam) said: If any Muslim who suffers some calamity says that which Allah has commended him: "We belong to Allah and to Him shall we return: O Allah, reward me for my affliction and give me something better in exchange for it", Allah will give him something better than it in exchange . 2.Having strong faith that Allah will grant good reward for the previous miscarriages. The Prophet (Sallallahu Alaihi wa Sallam) said, Indeed the miscarriage pulls its mother to the paradise if she remains patient and hopes for reward from Allah. [ Ahmad , Ibn Majah , Sheikh al-Albani classified it as Sahih] 3. Having good faith and trust in Allah, Allah Says (interpretation of meaning): {…And whosoever fears Allah and keeps his duty to Him, He will make a way for him.}[65:2]. The Prophet (Sallallahu Alaihi wa Sallam) said: I am just as My slave thinks I am. [Reported by Imams Al-Bukhari and Muslim ]. There is another narration that reads: I am just as My slave thinks I am; if he thinks of good he will get that, if he thinks of me bad, he will get it. [ Ahmad , Sheikh al-Albani considered it as Saheeh] 4.Keep on reciting of the morning and evening supplications and seeking cure by reciting verses of Qur'an and supplications that are proved in Sunnah. 5.Being content with what Allah has decreed. Know that whatever is to strike a human being will never miss him, and whatever is to miss him will never strike him. Allah Says (interpretation of meaning): {No calamity befalls, but by the Leave [i.e. Decision and Qadar (Divine Preordainments)] of Allah, and whosoever believes in Allah, He guides his heart [to the true Faith with certainty, i.e. what has befallen him was already written for him by Allah from the Qadar (Divine Preordainments)]…}[64:11]. The Prophet (Sallallahu Alaihi wa Sallam) said: If you spend gold equal to Uhad (a mountain on the outskirts of Madinah in Allah's Cause Allah will not accept it unless you believe in Divine decree and believe that whatever to strike you will never miss you, and whatever will miss you will never strike you. If you die on a faith other than this faith you will enter the Fire. [ Ibn Majah , and Sheikh al-Albani consider it as sound]. 6.Keep on doing good deeds and abstaining from committing sins since obedience of Allah brings goodness while His disobedience deprives one of blessings. Allah Says (interpretation of meaning): {…and fear Allah that you may be successful.}[2:189]. The Prophet (Sallallahu Alaihi wa Sallam) said: When a person desires to make sex with his wife, he should say: 'By the name of Allah, O' Allah, protect us from Satan and keep the Satan away from what you bestow upon us'. 7.Reciting the Du'a which is recited before having intercourse as proved in a Hadith that the Prophet (Salla Allahu Alaihi Wa Sallam) said, When a person desires to have sex with his wife, he should say: 'By the name of Allah, O' Allah, protect us from Satan and keep the Satan away from what you bestow upon us'. Ibn Abbas reported The Prophet Muhammad (Blessings and peace of Allah be upon him) said: When a person makes sex with his wife and recites this Du'a (supplication), If Allah gives a baby, Satan will not harm him/her. 8.Using legal Ruqiya as there might have been some Sihr (magic). But beware of charlatans. Finally, besides doing what we mentioned do not neglect consulting medical doctor, they may help you, Insha Allah. May Allah grant you pious children. Allah knows best.