That can have devastating consequences if untreated - the bacterial infection has caused six recent stillbirths, mostly among young Māori women.
Nicholas Jones investigates the highly preventable tragedies and finds gaps in the health system are putting families at risk.
The call came when Sarah was out of town. A nurse had results from a blood test taken at her first antenatal appointment.
"It's positive for syphilis," he told her. There wasn't much more information; he didn't know enough, and said the GP would call soon.
A panicked Google search brought up what could happen when the disease infects unborn child: stillbirth, miscarriage, deafness, deformity and brain damage.
The 29-year-old's world, so recently and thrillingly widened, collapsed. Her hands shook as she rang home.
"You hear the word 'syphilis' and you go into a tailspin," her mother, Jenny, recalls of the phone call.
"I mean, that's what Henry VIII had - you don't expect it ... I went straight to the internet as well, and saw what she was seeing. I was distraught."
Health systems 'caught off-guard'
An early recorded outbreak of syphilis, a sexually transmitted bacterial infection, was among French soldiers invading Naples in 1495. The "great pox" could be deadly and caused huge boils, rotting flesh and was blamed for monarchs' madness.
The disease afflicted as many as one in 10 Londoners by the start of the 20th century, but the use of penicillin after World War II all but eradicated syphilis in developed countries such as New Zealand.
However, it's now spreading at alarming rates in Western countries, a resurgence turbo-charged by dating apps, more international travel, poverty, and health services unprepared for an epidemic most thought consigned to medical history.
In New Zealand, infections have surged by about 540 per cent over five years, to around 528 reported cases last year - likely a fraction of the true number of infections.
Our amputation shame: Thousands of NZers lose limbs to diabetes
Men who have sex with men account for about 70 per cent of recorded cases, but the epidemic is infecting more straight women, mostly of childbearing age.
In 2016 that produced the "trainwreck" sexual health clinicians have long warned of - a stillbirth caused by syphilis passed on from the mother.
It proved no outlier; in the following two years there were nine mother-to-child infections, five of which caused stillbirths (some remain under investigation). There are already another three suspected "live birth" cases this year.
The toll is probably greater, because deaths are only recorded if they happen after 20 weeks' gestation. Earlier miscarriages are likely to have been caused by syphilis, but don't show in official statistics.
A Herald investigation can also reveal a loophole meant mother-to-child infections weren't caught, despite parents doing all the tests asked of them.
Most women who lost babies are poor and Māori. The youngest to lose a child was a 16-year-old who suffered a stillbirth at 27 weeks' gestation.
There is hope. At a recent Auckland sexual health conference Malaysian delegates celebrated the World Health Organisation declaring their country free from congenital syphilis. Thailand, Cuba and Belarus have had similar success.
Meanwhile, babies die here, in Australia, Britain and the United States.
"As a society and healthcare system we have been caught off-guard," said Dr Massimo Giola, clinical lead for the Bay of Plenty sexual health service, a region where a death occurred.
In Hamilton, Dr Jane Morgan said the deaths were preventable.
"This should not be happening in our country. In medicine we call it a sentinel event. It's meant to be a disruptor, it's meant to make you sit up and go, 'What's going on?'
"It's something massive happening. And we all need to work out what we're going to do to fix it."
'You're looking for signs of abnormality'
Sarah couldn't reach her GP by phone and so drove to Auckland for an appointment. She left believing her pregnancy was at extremely high risk. A midwife also knew little, leaving for 10 minutes and returning with printouts from the internet.
She found a clear course of action only after she took herself to the Hamilton Sexual Health clinic, and she and her partner got penicillin injections.
About half of syphilis infections don't result in symptoms, and condoms aren't always a protection because of oral sex transmission.
Sarah and her mother Jenny asked for their real names to be withheld because of the stigma associated with STIs. It's unclear where the infection came from, Jenny said, but a former partner may have been unfaithful.
Anxiety and worry marked the pregnancy and birth, which Jenny attended.
"You just don't know whether you're going to have a live baby at the end of it all. You're looking for signs of abnormality, but you can't see anything, really."
Blood was taken from screaming baby. Everything seemed fine, but a couple of weeks later a new GP phoned and said the test was positive.
Jenny was shopping in Newmarket's Flo & Frankie when her daughter rang. She stood among the dresses and tried to decipher the sobs.
"I was going, 'What's wrong? What's wrong?' I couldn't hear her properly. I thought she'd dropped her baby. I tried to reassure her while I was having a meltdown myself. I was nearly throwing up."
Reeling, Jenny searched the internet for help. She found contact details for Morgan, the clinical director of the Waikato District Health Board's Hamilton Sexual Health.
"And bugger me if she didn't call back within 10 minutes. I sat on a seat in Nuffield St, listening to Jane explain what this positive blood test for the baby meant."
It was reassuring news; the test had probably picked up antibodies produced by Sarah, and there was a tiny chance the disease had been passed on.
Jenny's friends waited at a bar across the road, ready to toast her new grandchild.
"They could see me on the phone, they were all happy and had little cardigans. Just a few moments before I had thought, 'Jeepers, we mightn't need any gifts for this baby.' We didn't know where things were headed."
'A blatant failure'
Sexual health is known as a Cinderella service - last to the ball when limited health dollars are dished out. There are only about nine full-time sexual health specialist positions across New Zealand's 20 DHBs, and areas without specialists include those where babies have died from syphilis.
Documents show the Association of Salaried Medical Specialists, NZ Aids Foundation and the Royal Australasian College of Physicians warned about threadbare and reduced services, with the latter labelling the official response to the epidemic "totally inadequate".
The few specialist positions means it's often GPs, nurses and midwives at the coalface. However, many lack training or are unaware of the building crisis.
Sexual health training at medical school generally consists of a lecture on each of the most common STIs, Morgan said, and until about 10 years ago students were told syphilis was rare and a test reaction would probably be a false positive.
Test results can be tricky to interpret - as seen in the confusion surrounding Sarah's care - and sexual health clinics encourage GPs to call for advice.
"GPs aren't aware that syphilis is really, truly back," Morgan said. "People aren't having tests. We're not doing enough testing, not even vaguely."
The Herald has confirmed mother-to-child syphilis cases where tests weren't acted on, or properly carried out.
In the Bay of Plenty, a woman was seen by both a GP and a midwife during pregnancy, but didn't have adequate testing, despite her having genital ulcers, a possible symptom. Her child was stillborn.
An Auckland woman had a baby with untreated syphilis, despite testing positive during antenatal screening. She wasn't informed or treated before delivery. Fortunately, the baby was okay.
At a health conference last year Dr Massimo Giola told attendees the outbreak and deaths "are the demonstration of the blatant failure of 10 years of policies of devolution of sexual health to primary care without support and training of the primary care workforce".
Giola told the Herald his comments were a call for action to a health sector that had put sexual health "on the backburner". There were encouraging signs of an official response, he said, including the development of a national syphilis action plan.
But it wasn't just a problem for politicians, health workers and bureaucrats. Giola was born in Italy, and moved here in his 40s. One observation of his adopted home: a discomfort with talking about sex.
"Sexual health, sex education in schools, teenage pregnancy rates, abortion law reform, domestic violence, transgender issues - there's a range of issues that as a society we should be debating more openly."
Test twice to save life
All women should get a blood test for syphilis and HIV when they start antenatal care.
Both Giola and Morgan want that repeated in the third trimester, to catch cases where infection happens during pregnancy. Syphilis can take up to three months to show in testing, so retesting also acts as a backstop for infection too early to show up in the first blood test.
Under existing guidelines, "at risk" women - who have an STI or sex with a new partner in pregnancy - should be retested. However, asking such questions can be awkward, particularly if pressed for time. There's also the possibility the father has other sexual partners.
"We never know," says Giola. "The last congenital case we had in the Bay of Plenty was essentially a failure of screening in pregnancy, because the woman was not deemed to be at risk, and so was not retested."
The Herald understands another case happened after a woman fell pregnant soon after meeting her partner. An infection didn't show up in the blood test done early in the pregnancy.
Closing the testing loophole would cost about $270,000 a year (a test costs $4.50, about 60,000 women give birth annually). Even if one case was prevented it would be worth the money, Morgan said.
"In Australian outbreak areas they are going to screen every woman five times ... making something universal takes away the stigma from the people with the most risk and says, 'This is normal. This is for everyone.'"
Asked about third-trimester testing, a Ministry of Health spokeswoman said a professional body, the NZ Sexual Health Society, will finalise national guidelines for treating maternal syphilis by the end of the year.
"We anticipate these guidelines will include a recommendation regarding third-trimester screening."
Most women losing babies are Māori and among the bottom 10 per cent in terms of deprivation.
The deaths are history repeating; the first cases in New Zealand followed the arrival of James Cook's ship the Endeavour in 1769, with Māori women infected by his crew having miscarriages and giving birth to babies with deformities.
Jean Te Huia, chief executive of the 310-member Ngā Maia Māori Midwives Aotearoa, said an outbreak 250 years later was an indictment on society.
"To measure how well a country does for its citizens, you look at the poorest and most vulnerable. And they would have to be Māori, indigenous children."
Te Huia, who was recently acknowledged by Health Minister David Clark for her volunteer work, said there was a desperate lack of free and accessible sexual health services. That reflected a "shame and blame" attitude towards people needing help.
"It's like everything to do with women's health - it gets sidelined and put to the back ... we can't blame it on this Government or the last. It has been happening successively for a number of Governments."
Dr Niki Stefanogiannis, the ministry's deputy director of public health, said mother-to-child syphilis "reflects international trends, and reinforces a global need for action".
"Most cases of congenital syphilis in 2017 and 2018 presented to health services very late - either in labour or in the week before labour.
"We're looking at the best ways to provide advice to lead maternity carers and to develop educational resources for pregnant women and their LMCs."
All 20 DHBs have been told to work on local action plans to combat syphilis, but initiatives must be funded from within existing resources - and year-to-March results show DHBs were in the red by $264 million.
In contrast, the Australian Government put up $21m over four years to try to reverse the growing number of syphilis cases.
Morgan grew up in Northern Ireland during the Troubles, a background she said made her attuned to "othering" language and attitudes - the view, direct or implicit, that people are to blame for their own suffering.
"We had two tribes. It was them and us. That's how you start dehumanising, and seeing them as less worthy and not being worth a fair crack."
She sees the same attitude here; that people are too lazy to get help, when clinic hours are when they're at a desperately needed job. Or they're 17 and don't have any vehicle or petrol money to drive from over an hour away. Or a shortage of midwives means none is available.
Many of the people who come through the clinic's doors are living on the edge, she said, and disease feeds on misery fuelled by insecure housing, families broken by violence, drugs and jail, and evictions from state houses.
"Each day when I start my clinic I have a big pile of Mental Health Foundation postcards on my desk ... the condom use and protecting your future fertility and avoiding infection, that's all about future health. Whereas the drug-taking and unsafe sex - that's comfort for today."
'We were lucky'
Sarah's baby will have a final blood test at six months, but there are no signs of health problems. The family - middle-class and Pākehā - know they were lucky, and that their backgrounds may have made the difference.
"Two things happened that may not happen to a large proportion of the community," Jenny said of her daughter's experience. "She took herself to the sexual health clinic, knowing she needed extra help after her original blood-test result. And I contacted Jane [Morgan].
"That was us being proactive. I work in education with underprivileged families, dysfunctional families. And I can tell you right now that many people in those situations don't know that you can seek help.
"We were lucky. There are other people who have lost babies."