The fuqaha'(jurists) have inferred that the angel of life blows the soul into the foetus after it is 120 days old, so without doubt, terminating the life of a foetus when it is over 4 months old is definitely murder (speaking in general terms).
This is based on the hadeeth:
عن أبي عبد الرحمن عبد الله بن مسعـود رضي الله عنه ، قال : حدثنا رسول الله صلي الله عليه وسلم – وهو الصادق المصدوق - :( إن أحـدكم يجمع خلقه في بطن أمه أربعين يوما نطفه ، ثم يكون علقة مثل ذلك ، ثم يكون مـضغـة مثل ذلك ، ثم يرسل إليه الملك ، فينفخ فيه الروح ، ويـؤمر بأربع كلمات : بكتب رزقه ، واجله ، وعمله ، وشقي أم سعيد ؛ فوالله الـذي لا إلــه غـيره إن أحــدكم ليعـمل بعمل أهل الجنه حتى ما يكون بينه وبينها إلا ذراع فيسبق عليه الكتاب فيعـمل بعـمل أهــل النار فـيـدخـلها . وإن أحدكم ليعمل بعمل أهل النار حتي ما يكون بينه وبينها إلا ذراع فــيسـبـق عليه الكتاب فيعمل بعمل أهل الجنة فيدخلها ) رواه البخاري [ رقم : 3208 ] ومسلم [ رقم : 2643 ]
On the authority of Aboo 'Abd ir-Rahmaan 'Abdullaah ibn Mas'ood (radiAllaahu anhu), who said: The Messenger of Allaah (sallAllaahu alayhi wa sallam) and he is the Truthful, the Believed, narrated to us:
"Verily the creation of each one of you is brought together in his
mother's womb for forty days in the form of a nutfah (a drop), then he
becomes an 'alaqah (clot of blood) for a like period, then a mudghah
(morsel of flesh) for a like period, then there is sent to him the
angel who blows his soul into him and who is commanded with four
matters: to write down his rizq (sustenance), his life span, his
actions, and whether he will be happy or unhappy (i.e. whether or not
he will enter Paradise).
By the One, other than Whom there is no deity, verily one of you
performs the actions of the people of Paradise until there is but an
arms length between him and it, and that which has been written
overtakes him, and so he acts with the actions of the people of the
Hellfire and thus enters it; and verily one of you performs the
actions of the people of the Hellfire, until there is but an arms
length between him and it, and that which has been written overtakes
him and so he acts with the actions of the people of Paradise and thus
he enters it. [Narrated by al-Bukhaari and Muslim.] "
This also goes hand in hand with what Allah revealed regarding this matter:
“And verily We did create man from a quintessence (of clay). Then We
placed him (as a drop of sperm) in a place of rest, firmly fixed. Then
We made the sperm into a clot of congealed blood. Then of that clot We
made a (foetus) lump. Then We made out of that lump bones and clothed
the bones with flesh.THEN We developed out of it another creature (by
breathing life into it). So blessed be Allah, the most marvellous
Creator” (Surah al-Mu’minun 12-13-14).
The fuqaha’ of the Sunni Islamic madhhabs have agreed that it is haram to kill the foetus after the soul has been breathed into it, i.e., after 120 days have passed since conception. It is not permissible to kill it under any circumstances except when continuing the pregnancy would lead to the mother’s death.
The differences among the fuqaha’ have to do with abortion in the period before the soul is breathed into the foetus. After the soul is breathed into it, the fuqaha’ are agreed that the foetus has become a human being and a soul which has dignity and is to be respected.
Allah says (interpretation of the meaning):
“And indeed We have honoured the Children of Adam…”[al-Israa’ 17:70]
And He says (interpretation of the meaning):
“… if anyone killed a person not in retaliation of murder, or (and) to
spread mischief in the land — it would be as if he killed all mankind,
and if anyone saved a life, it would be as if he saved the life of all
Hence, killing an unborn child after soul has been breathed into it without any legally permissible reason would be considered as murder.
If you are interested in know the Ruling on abortion at different stages of the foetus, check here.
More Reference:Abortion after the soul has been breathed into the foetus