[matthewh (OP)]

Gregory has asked us:

I want to know how accurate ultrasounds are to assess the sex of an unborn baby? The reason behind this question is my wife had given birth to a baby boy on the 28th March 2019, but was told during her 20-week scan that we were supposed to have a baby girl.

Also, I wanted to know how common are over-due babies and whether or not there are any major health risks with babies born after their due date?

Let's congratulate the new family with some answers to their questions!

[alancalverd]

Congratulations and best wishes indeed!

I was late arriving and have every intention of being late for my funeral. Due dates are at best a guess, at worst an example of administration interfering with clinical decisionmaking. Midwife sister-in-law said "if there's nowt wrong, they come when they're ready."

Having worked in imaging physics since the early days of ultrasound, I never understood the ritual pregnancy scan. It just seems to add worry and confusion to a process that has been going on for billions of years without third party intervention.

A good sonographer should be able to point out a fetal penis at 20 weeks if the baby cooperates, but absence of evidence is not evidence of absence, and they don't always cooperate - why should they?  It's more difficult to invent one that isn't there, so the error tends to be onesided.

There's much to be gained from ultrasound scanning if there is a suspected problem, but as this story shows, unnecessary "routine" scanning just wastes time and resources at best, and makes life difficult at worst. Most sonographers I have met, agree with me: if it ain't busted, don't fix it.

Cynical or realistic? I have 4 children and my wife was a nurse. The sex of each one just added an element of surprise to the joy of their birth. 

[evan_au]

All embryos start developing as females, and masculinisation only really starts about week 10, and takes a while to become more obvious.
See: https://en.wikipedia.org/wiki/Sexual_differentiation_in_humans#Timeline

Sometimes it's even hard to tell at the time of birth...
...and some cases are not discovered until puberty (or the lack of it)

Fortunately, this affects much less than 1% of births.
See: https://en.wikipedia.org/wiki/Intersex

Bit it sort of calls into question the whole "gender reveal party" thing...