Ms Penhall's blood is Rh-negative, meaning it does not have the rhesus factor; her four pregnancies have been Rh-positive.
She was probably "sensitised" during Samson's birth, from a small amount of his blood entering her own bloodstream and causing her immune system to make "anti-D" antibodies against Rh-positive blood.
A paediatrician noticed the incompatibility soon after the birth and Ms Penhall was given an injection of anti-D immunoglobulin, a blood product that usually stops an Rh-negative woman from being sensitised. But she wasn't tested to see if the dose was big enough and the treatment failed.
Around 5000 Rh-positive babies, nearly 9 per cent of all births, are born to Rh-negative women in New Zealand each year. Around 750 of these babies need treatments, such as blood transfusions both in the uterus and following birth, and ultraviolet light therapy.
From 2007 to 2013, two babies died in late pregnancy or soon after birth from rhesus incompatibility. Once the mother is sensitised, antibodies from her blood cross the placenta and destroy red blood cells in an Rh-positive fetus, causing anaemia - and jaundice, which can lead to brain damage.
Ms Penhall went on to have a miscarriage at 20 weeks and her third baby, Elijah, now 15 months, had to be delivered by caesarean around eight weeks premature when his anaemia spiked, among other problems.
"I said 'The baby has to come out'. He wouldn't have survived another few hours."
He received several blood transfusions and seven weeks of UV-light therapy.
Ms Penhall is now 33 weeks pregnant with a baby girl who had received six intra-uterine blood transfusions for anaemia by last week. Two more were booked ahead of her caesarean delivery expected at 36 weeks. The baby will need UV-light therapy and possibly more transfusions following birth.
This costly, high-tech care and Ms Penhall's miscarriage could probably have been avoided, she believes, if New Zealand, like Australia and Britain, had a policy of routinely offering the anti-D injection to Rh-negative women during pregnancy unless they were already sensitised.
She speaks to groups and is pressing the Government to make the dosage test routine following at-risk births - an audit found big variations between health districts - and to provide more education on rhesus incompatibility.
Her campaign on the preventive injections reflects the recommendations of the Blood Service and the transtasman College of Obstetricians and Gynaecologists, but it is not policy for the Ministry of Health.
Blood Service specialist Dr Krishna Badami said, "While it may not be 'official' policy, some practitioners offer [the preventive injections] to those who are eligible."
With colleagues, he wrote in the New Zealand Medical Journal that their Christchurch-based study had found around half of rhesus sensitisation cases involved a failure to comply with medical guidelines.
"Better adherence to this may reduce incidence of sensitisation. The incidence is three times higher than it might be if a [routine antenatal anti-D preventive treatment] programme was also in place."
The latter finding is broadly in line with a review by the Cochrane Collaboration, a highly respected international advocate for evidence-based medicine.
But the ministry's chief adviser on child and youth health, Dr Pat Tuohy, said the Cochrane evidence was too weak to require big changes in clinical practice.
In an email, Ms Penhall was told by the office of her local MP Dr Jonathan Coleman, before he became Health Minister, that the College of Midwives "does not agree that there is sufficient weight of evidence to support the effectiveness of giving women anti-D during pregnancy".
Dr Tuohy said preventive injections of anti-D were trialled in the Manawatu region in 2010 but there were "logistical issues" and "some areas of current practice that would need to be improved to ensure that all women were getting appropriate prevention and treatment for Rh disease".
There was no consensus on the issue and without definitive evidence, "practitioners need to make a clinical decision in conjunction with the patient".
The Herald asked the College of Midwives about its reported opposition to the preventive treatment.
Midwifery adviser Lesley Dixon referred to the feasibility trial and said: "The college was primarily concerned with the roll-out process in regards to equity of access which can be problematic in rural New Zealand. [Anti-D] is a blood product and as such has particular requirements to ensure safety during storage, prescription and administration."
Dr Peter Stone, the professor of maternal fetal medicine at Auckland University, supports, as one option, giving anti-D during pregnancy, at 28 and 34 weeks, to all Rh-negative women who are antibody negative.
The alternative - which "should have been available in New Zealand ages ago" - was to test the genotype and blood group of the fetus of all Rh-negative women and give anti-D injections only to mothers of an Rh-positive fetus.
What is the disorder?
Rhesus (Rh) blood type incompatibility. The mother is Rh-negative and the fetus Rh-positive. The woman's first of these babies is unaffected, but during the birth, some of the baby's blood can get into the mother's bloodstream and "sensitise" her immune system. She begins producing "anti-D" antibodies against Rh-positive blood and this can cause a miscarriage in a later Rh-positive pregnancy, and the need for blood transfusions and ultraviolet light therapy if the baby survives.
How many babies does it affect?
Around 5000 Rh-positive babies a year are born to Rh-negative mothers. Around 750 babies a year need treatment. Two deaths of a newborn or fetus beyond 20 weeks' gestation were attributed to the disorder from 2007 to 2013.
What can be done about it?
Giving an affected woman an injection of anti-D immunoglobulin within 72 hours of the first baby's birth, or after a bleed or other potentially sensitising events in pregnancy, can destroy any of the baby's blood in her bloodstream and usually prevents her being sensitised. Some medical groups, but not the Health Ministry, recommend the injections be offered as a preventive treatment to Rh-negative women during pregnancy unless they are already producing their own antibodies.
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