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Grace Meachim, 32, and James Meachim, 34, decided they would stop at three children, but little Sienna Meachim - born six months ago - had other plans.
Sales advisor Grace hadn't the faintest idea that she was around 37 weeks pregnant; she had no discernible bump, had regular periods, and was taking her pill "religiously".
Waking up in the early hours of the morning to take a paracetamol and brew a cuppa as she was feeling poorly, Grace suddenly felt the urge to go to the loo.
But as she sat down on the toilet, the mum felt her waters break and let out a scream.
Grace, of Littlehampton, West Sussex, said: "Within seconds of getting to the toilet, my waters went. It frightened me so much I screamed.
"It was so surreal. I hadn't had any build-up to it at all, no pain, nothing.
"When I'd gone to the loo, I didn't feel like I was going into labour at all. After three previous pregnancies, I know what that feels like.
"There was a big gush then there was pressure and a mass down there.
"I knew then what was happening and I went into shock. All I can remember thinking is 'oh my god, is this really happening?'.
Husband James and their eldest son, Tyler, 13, rushed to the bathroom, afraid she was "being attacked".
But when Grace shouted to them that there was "something coming" they phoned for an ambulance.
No sooner had he reached the 999 operator, James had to make a mad dash back to the loo to catch his baby daughter as Grace gave birth.
"He told the operator 'I think it's a baby but she's not pregnant'," said Grace.
"As soon as he said that he had to throw the phone to Tyler because I screamed 'quick, catch it'.
"Within seconds James had his hands down the toilet catching the baby, it all happened so quickly.
"It was very strange, it was like something out of a movie.
"I just kept saying 'I'm sorry' to my husband because we'd been dead set against any more children.
"James was waiting to be booked in for a vasectomy and I'd planned to up my hours at work so we could start saving for a mortgage."
When paramedics arrived, Grace and her newborn were taken swiftly to hospital.
Little Sienna weighed in at 7lbs 5.5oz. Happily, both mum and baby were given a perfect bill of health and sent home.
Grace and James are thrilled with Sienna's "extra special" arrival but are adamant there will not be a baby number five.
"She was perfect and a really good size, I have no idea how I hid that," said Grace.
"She was such a nice surprise, she is that little bit extra special to us. And we definitely know she's here now, she is really coming out of her shell.
"We wouldn't change it but it's definitely no more kids now. I've told James he needs to chase up his snip as soon as lockdown is over."
Sienna was born around 4am on 5th December 2019, much to the disappointment of the ambulance crew who thought she would be their first ever home birth.
Grace's and James's friends and family were stunned by the news and have rallied around the new member of the family.
"When James called my mother-in-law she just said 'congratulations' and hung up then called back seconds later to say 'sorry, I just registered what you said'. Everyone was so shocked," said Grace.
"I announced it on Facebook as like 'surprise, look what we have done' and our friends just couldn't believe it."
Sienna's sudden arrival is a particularly big adjustment for toddler Blake, who went to bed as the baby of the family and woke up as a big brother!
All of Sienna's siblings are besotted with their little sister.
Grace said: "He [Blake] went to bed and then when he woke up I wasn't there and I'd had another baby. But by the time we brought Sienna home he was like a different child, stroking her and playing with her.
"They have all got their own unique bonds with her, they're incredible with her."
Leading up to Sienna's birth, Grace had been having regular periods and taking her contraceptive pill.
However, the mum had noticed a 'slight lump' in her tummy and took three at-home pregnancy tests - all with negative results.
Added to that, in her three previous pregnancies, Grace had very visible bumps and experienced the usual pregnancy symptoms such as morning sickness.
Grace visited her GP in September, at five months pregnant, and the doctor conducted another pregnancy test. Incredibly, the result also came back negative.
Grace was diagnosed with a recurring cyst, as she had suffered one the previous year which dissipated on its own.
The mum was booked in for an ultrasound of the 'cyst' in January 2020 but Sienna was born a month before.
Grace said: "With all my pregnancies before Sienna I knew straight away. With my first and third, I had huge bumps.
"But with Sienna, I had no signs, no symptoms, nothing. All my pregnancy tests were negative, everything was regular throughout.
"The year before I had had a cyst that eventually dispersed so when I went to the doctor again last year about the lump, she checked it out and said it felt the same as last time.
"Because I was healthy otherwise, they didn't want to do surgery to remove it but said I'd probably have a scan in the new year.
"I was happy with that. I didn't want to take the time off work for surgery and I wasn't in any pain so I was happy to wait. That was all just a few months before Sienna arrived."
What a sweet - if very surprising - arrival! Congratulations to the Meachim clan.
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