Isn’t it hardship to compel the woman who gets her period before Maghrib to make up the fast? And does she miss out on the reward for that?

Firstly:

The things that invalidate the fast are mentioned in the religious texts, the Qur’an and Sunnah, and the scholars are unanimously agreed on many of them. When it comes to these things that invalidate the fast, it is all the same whether there is a small amount or a large amount. Whoever eats a single grain of rice or drinks a single drop of water has broken his fast according to the consensus of the Muslims, and it cannot be said that this is something small that does not nourish a person. The reason for that – and Allah knows best – is that Allah, may He be exalted, has caused the general rulings which the Muslims need to be based on visible things, and He has set out guidelines for them, so that there will be no confusion about them.

The acts of worship that are prescribed in Islamic teachings, such as prayer, fasting and so on, have set limits which Allah, may He be glorified and exalted, has defined clearly in His Book, and He has forbidden us to transgress those limits. He says (interpretation of the meaning):

{These are the limits of Allah, which He makes clear to a people who know} [al-Baqarah 2:230]

{These are the limits of Allah, so do not transgress them. And whoever transgresses the limits of Allah – it is those who are the wrongdoers} [al-Baqarah 2:229].

Menstruation invalidates the woman’s fast according to scholarly consensus, if it occurs during the least part of the time during which she is enjoined to fast, from the break of dawn until the setting of the sun.

An-Nawawi (may Allah have mercy on him) said:

If a woman menstruates during part of the day, her fast is invalidated – with no difference of scholarly opinion concerning that – and she has to make it up. The same applies to bleeding following childbirth (nifas); it invalidates her fast, with no difference of scholarly opinion concerning that."(Al-Majmu‘  6/385).

For more information, please see the answer to question no. 38027 .

Secondly:

With regard to that being a cause of hardship, there is no doubt about that, but it is mild and bearable hardship, because this does not happen repeatedly to the woman. Rather it is only one day in Ramadan – if it happens at all – and she has a whole year ahead of her in which to make up this one day. So this is mild hardship that does not lead to the waiving of that obligation.

All the obligations prescribed in Islam involve some element of hardship.

Al-Qarafi (may Allah have mercy on him) said in al-Farq ar-Rabi‘ ‘Ashara (1/281):

Hardship falls into two categories:

The first category is that which is essential for doing an act of worship, such as doing wudu’ and ghusl when it is cold, or fasting on a day that is long, or risking one’s life in jihad, and so on. This category does not dictate that there should be any concession regarding the act of worship, because it is prescribed as being essential to it.

The second category is hardship that is not essential to doing an act of worship. This is of three types.

The first type is the most severe, such as fear for life, limb and faculties. In such cases, concessions regarding the act of worship become necessary.

The second type is the least severe, such as very mild pain in the finger. In this case, doing the act of worship is more important than warding off this hardship, because of the great importance of acts of worship and the mildness of this hardship.

The third type is hardship that falls between these two extremes. That which is closer to the extreme type dictates that there should be a concession regarding the act of worship, and that which is closer to the mildest type does not dictate that. Regarding that which falls in the middle, it varies according to the category to which it is closer. Based on the two guidelines mentioned above, fatwas may be issued regarding hardship in doing acts of worship. End quote.

Thus it becomes clear that not every case of hardship dictates that obligations are to be waived.

Thirdly:

If a woman starts the day fasting, then gets her period, her fast is rendered invalid and she has to make up that day.

Nevertheless, what appears to be the case – and what is appropriate to the generosity and immense grace of Allah – is that Allah will record for her the reward for what she fasted of that day, because she fasted in obedience to Allah’s command and she broke her fast in obedience to Allah’s command, and because He excused her from fasting.

Allah, may He be exalted, says regarding one who set out to migrate for the cause of Allah and His Messenger, then died before reaching his destination:

{And whoever leaves his home as an emigrant for the cause of Allah and His Messenger and then death overtakes him – his reward has already become incumbent upon Allah. And Allah is ever Forgiving and Merciful} [an-Nisa’ 4:100].

Imam al-Jassas (may Allah have mercy on him) said: This verse tells us that the reward of one who migrates for the sake of Allah and His Messenger is due, even if he does not complete his migration.

This indicates that if someone sets out to do something in order to draw close to Allah, Allah will reward him in accordance with his intention and effort, even if he is prevented from completing it, just as Allah has made due the reward of the one who sets out to migrate for His sake, even if he does not complete his migration."(Ahkam al-Qur’an  2/314).

Shaykh as-Sa‘di (may Allah have mercy on him) said: { And whoever leaves his home as an emigrant in the cause of Allah and His Messenger } that is, migrating for the sake of his Lord and seeking His pleasure, out of love for His Messenger and in support of the religion of Allah, not for any other purpose

{and then death overtakes him} by being killed or otherwise

{his reward has already become incumbent upon Allah} that is, he will attain the reward of the muhajir (emigrant) who achieved his goal of migration by being guaranteed a complete reward by Allah. That is because he intended and decided, and he took the first step and started to take action. By Allah’s mercy towards him and others like him, He will give them the reward in full, even if they did not complete the action, and He will forgive them for whatever shortcomings occurred in terms of migration and otherwise."(Tafseer as-Sa‘di  196).

The jurists have mentioned a case similar to that asked about here, namely someone who sets out to do Hajj, then dies before completing the rituals.

Al-Mulla ‘Ali Qari (may Allah have mercy on him) mentioned the difference of scholarly opinion regarding a pilgrim who dies on the way to Hajj: from where should Hajj be done on his behalf? Then he said: This is based on their difference of opinion regarding one who sets out to do Hajj on his own behalf and dies on the way [to Makkah]. He may issue instructions [before he dies] that Hajj should be done on his behalf setting out from his house, according to Abu Hanifah. According to both of his students – and it is preferable – it should be done from the place where he died, for his journey was not rendered invalid by his death, because Allah, may He be exalted, says (interpretation of the meaning):

{And whoever leaves his home as an emigrant to Allah and His Messenger and then death overtakes him – his reward has already become incumbent upon Allah} [an-Nisa’ 4:100].

And because the Prophet (blessings and peace of Allah be upon him) said: “Whoever sets out to do Hajj and dies, Allah will record for him the reward of the pilgrim who does Hajj until the Day of Resurrection. And whoever sets out to do ‘umrah and dies, the reward of the pilgrim who does ‘umrah will be recorded for him until the Day of Resurrection. And whoever sets out to fight as a warrior in Allah’s cause and dies, the reward of the warrior will be recorded for him until the Day of Resurrection.” Narrated by at-Tabarani in his Mu‘jam and by Abu Ya‘la al-Mawsili in his Musnad."(Fath Bab al-‘Inayah bi Sharh an-Niqayah  3/189).

Conclusion:

What appears to be the case is that if a woman gets her period when she is fasting, she will be rewarded for what she did of fasting, just as she will be rewarded for her intention to complete the fast, were it not for that excuse.

And Allah knows best.