The most common and most likely answer to this crossword clue is the 8 letter word PLACENTA
I've seen this crossword clue in The Times
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The straight part
We know that "Where mum joined baby" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where mum joined baby = placenta.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PLACENTA mean?
PLACENTA noun- A vascular organ in mammals, except monotremes and marsupials, present only in the female during gestation. It supplies food and oxygen from the mother to the foetus, and passes back waste. It is implanted in the wall of the uterus and links to the foetus through the umbilical cord. It is expelled after birth.
- It is an endocrine gland which secret human chorionic gonadotropin hormone.The HCG if detected in woman's urine then the pregnancy is confirmed.
- In flowering plants, the part of the ovary where ovules develop; in non-flowering plants where the spores develop.