Primarily, sex determination occurs at fertilization and depends upon the type of sex chromosome in the sperm that fertilizes an ovum. If it is an ‘X’ bearing sperm that fertilizes the ovum, the foetus is a female and if it is a ‘Y’ bearing sperm then the foetus is a male.
053.045► وَأَنَّهُ خَلَقَ الزَّوْجَيْنِ الذَّكَرَ وَالأنْثَى
053.046► مِنْ نُطْفَةٍ إِذَا تُمْنَى
“That He did create in pairs – male and female, from a seed when lodged (in its place).” [Al-Qur’an 53:45-46]
The Arabic word nutfah means a minute quantity of liquid and tumnaa means ejaculated or planted.
Therefore nutfah specifically refers to sparm because it is ejaculated. The Quran says-
075.037► أَلَمْ يَكُ نُطْفَةً مِنْ مَنِيٍّ يُمْنَى
075.037► Was he not a drop of sperm emitted (in lowly form)?
075.038► ثُمَّ كَانَ عَلَقَةً فَخَلَقَ فَسَوَّى
075.038 ►Then did he become a leech-like clot; then did (Allah) make and fashion (him) in due proportion.
075.039► فَجَعَلَ مِنْهُ الزَّوْجَيْنِ الذَّكَرَ وَالأنْثَى
075.039► And of him He made two sexes, male and female. [Al-Qur'an,-Surah Al-Qiyama- 075.037-039]