I am pregnant/nursing. Do I need to fast Tisha Bav this year (5779), being that it is a Nidche (pushed off)?
Tisha Ba’Av is a more stringent fast day and certainly one should try their absolute best to complete the entire fast.
The Shulchan Aruch (Orach Chayim 554:5) writes that even pregnant and nursing mothers fast on Tisha Ba’Av like how they do for Yom Kippur.
This year when it’s pushed off there is more room for leniency, if necessary. If difficult, don’t fast.
However, before eating you should make Havdalah.
For more on this topic, see halacha2go #718