When was Delphini born?

Question

Spoilers ahoy! [Go no further if you haven't read Harry Potter and the cursed child yet.]

In Harry Potter and the cursed child, Delphini states

Delphini: I am from the future. The child of Bellatrix Lestrange and you. I was born in Malfoy Manor before the Battle of Hogwarts. A battle you are going to lose. I have come to save you. (Act 4, Scene 11)

In the Deathly Hallows, Harry, Hermione and Ron are taken to Malfoy Manor, where there is no mention of her looking pregnant. Presumably, this means that the baby was born prior to that time- and, seeings as Malfoy is unaware of his cousin's birth- Bellatrix presumably had no contact with her family for 5 months prior (after the 4-month point the bump would have been more obvious.)

So, given this information, when was Delphini born (or concieved?)

Answer 1

The Harry Potter Wikia article on Delphini Diggory does a pretty decent job of extrapolating her likely birth-date from the available evidence.

Delphini (b. 1998^_?)

Delphi claims she was born at Malfoy Manor before the Battle of Hogwarts. However, when the trio are at Malfoy Manor in The Deathly Hallows Bellatrix is not said to be pregnant, meaning that, as there are only two months until the final battle, Bellatrix has already had the child.

Presumably there are spells and charms that could be used to conceal a pregnancy with semi-trivial ease, so it's not entirely true to say that her birth had to be in late 1998, but this seems the most likely position to take given how active she was when the trio arrived.

Answer 2

Well, where was Bellatrix at the night of the battle of the astronomy tower? Thinking she took "before" the battle liberally and was born around or just after this point. It fits because she missed the battle, she was at Malfoy manor, and Draco would have no idea because he was at Hogwarts. Bellatrix did teach Draco occlumency, but this could have been through fire, and if he saw her when she wasn't really pregnant, poor boy probably had enough to worry about and not have noticed (especially with clothing/charms). Also fits Bellatrix at Spinner's end "if I had sons, I'd happily give them to the dark lord."