أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ مُوسَى، عَنْ عُثْمَانَ بْنِ الْأَسْوَدِ، قَالَ: سَأَلْتُ , مُجَاهِدًا عَنْ امْرَأَتِي رَأَتْ دَمًا، وَأَنَا أُرَاهَا حَامِلًا، قَالَ: "ذَلِكَ غَيْضُ الْأَرْحَامِ اللَّهُ يَعْلَمُ مَا تَحْمِلُ كُلُّ أُنْثَى وَمَا تَغِيضُ الأَرْحَامُ وَمَا تَزْدَادُ سورة الرعد آية 8، فَمَا غَاضَتْ مِنْ شَيْءٍ رَأَتْ مِثْلَهُ فِي الْحَمْلِ".
Uthman bin Al-Aswad said: I asked Mujahid about my wife who had bleeding; I thought she was pregnant. He said: This is from the category of "Ghayz al-Arham" (Allah knows what the wombs contain, and the shrinking and swelling of the belly...) [الرعد 8/13]
So whatever decreases, the same amount increases in the pregnancy.