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Family Planning and Pregnancy: The Genetic Connection

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Familial Hypocalciuric Hypercalcemia Type 1 (FHH1) has a 50% chance of being passed to a child. During pregnancy, standard urine calcium tests are unreliable, so genetic testing is preferred. Genetic counseling is vital to manage newborn care and rare risks like NSHPT.

Key Takeaways

  • • FHH1 is an autosomal dominant genetic condition, meaning there is a 50% chance of passing it to your child with each pregnancy.
  • • Standard urine tests for FHH1 (CCCR) are highly unreliable during pregnancy and breastfeeding.
  • • Newborns of mothers with FHH1 may experience a temporary drop in calcium after birth that requires pediatric monitoring.
  • • If both parents carry a CASR gene mutation, the baby is at risk for a severe condition called Neonatal Severe Hyperparathyroidism (NSHPT).
  • • Genetic counseling is strongly recommended for anyone with FHH1 who is planning a family to coordinate testing and newborn care.

Because Familial Hypocalciuric Hypercalcemia Type 1 (FHH1) is a genetic condition, understanding how it is passed through families is a vital part of your care. This is especially true if you are planning a family or are currently pregnant, as the condition affects both the parent and the developing child in unique ways [1][2].

How FHH1 is Inherited

FHH1 follows an autosomal dominant inheritance pattern [3][4]. This means:

  • You only need one mutated copy of the CASR gene to have the condition [5][6].
  • With each pregnancy, there is a 50% chance that the child will inherit the mutation and have FHH1 [7][2].
  • There is also a 50% chance the child will inherit the normal gene and not have the condition [7].

Pregnancy: A Changing Environment

Pregnancy naturally changes how your kidneys handle calcium, which makes standard diagnostic tests difficult to interpret. The Calcium-to-Creatinine Clearance Ratio (CCCR)—the primary urine test used to diagnose FHH1—is considered unreliable during pregnancy and lactation [8][9].

In some women, the CCCR can double during pregnancy or drop significantly during breastfeeding, leading to potential misdiagnosis [8]. If you are pregnant and high calcium is discovered, doctors should rely on genetic testing or the calcium history of your relatives rather than urine ratios alone [8][9].

If you are pregnant, discuss your prenatal vitamins with your endocrinologist and obstetrician. You may need to adjust your intake of calcium and Vitamin D supplements to prevent your blood calcium from rising too high during pregnancy [8].

Newborn Considerations

If a mother has FHH1, the baby is exposed to high calcium levels in the womb. This can affect the baby’s own parathyroid glands [10][2].

  • Transient Neonatal Hypocalcemia: After birth, when the baby is no longer receiving high calcium from the mother, their suppressed parathyroid glands may take time to “wake up.” This can cause a temporary drop in the baby’s calcium (hypocalcemia). This is a treatable, temporary condition that resolves with standard pediatric care, usually requiring only short-term calcium supplementation [10][11].
  • Asymptomatic Hypercalcemia: If the baby inherits the FHH1 mutation, they will simply have the same mild, lifelong high calcium as the parent [10][2].

A Rare but Serious Risk: NSHPT

A very rare but severe condition called Neonatal Severe Hyperparathyroidism (NSHPT) can occur if a child inherits a mutated CASR gene from both parents (a homozygous mutation) [12][7].

In NSHPT, the baby’s calcium sensors are almost entirely non-functional, leading to dangerously high calcium levels and bone issues shortly after birth [12][13]. This condition is a medical emergency and requires immediate specialized care [7][2]. This is why it is critical for both partners to know their calcium status if one has FHH1.

The Value of Genetic Counseling

Because of these complexities, genetic counseling is strongly recommended for anyone with FHH1 who is considering children [10][1]. A counselor can help you:

  1. Understand the specific risks for your offspring based on your exact mutation [10].
  2. Coordinate testing for your partner and other family members [14].
  3. Prepare a plan with your medical team for monitoring the baby immediately after birth [9][1].
  4. Prevent future family members from undergoing unnecessary parathyroid surgeries [15][14].

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Frequently Asked Questions

How is FHH1 passed down to children?
FHH1 is an autosomal dominant condition, which means you only need one mutated copy of the CASR gene to have it. With each pregnancy, there is a 50% chance that your child will inherit the mutation and have FHH1.
Can I rely on standard urine tests for FHH1 while pregnant?
No, the standard calcium-to-creatinine clearance ratio (CCCR) urine test is unreliable during pregnancy and lactation. Because pregnancy changes how your kidneys handle calcium, doctors should rely on genetic testing or your family's calcium history instead.
How will my FHH1 affect my newborn baby?
Because your baby was exposed to high calcium in the womb, their parathyroid glands may take time to adjust after birth. This can cause a temporary, treatable drop in the baby's calcium levels. If they inherited the FHH1 mutation, they will eventually have mild, lifelong high calcium like you.
Should my partner be tested for FHH1 before we have a baby?
Yes, having your partner tested is highly recommended. If both parents carry a CASR gene mutation, the baby is at risk for Neonatal Severe Hyperparathyroidism (NSHPT), which is a rare but severe medical emergency requiring immediate specialized care after birth.
Why do I need to see a genetic counselor for FHH1?
A genetic counselor can help you understand the specific inheritance risks based on your exact mutation. They can also coordinate testing for your partner and help your medical team prepare a monitoring plan for your baby immediately after birth.

Questions for Your Doctor

  • • If I am planning a pregnancy, should my partner also be tested for CASR mutations to rule out the risk of NSHPT for our child?
  • • Since urine tests (CCCR) are unreliable during pregnancy, how will you monitor my calcium levels and the health of the baby?
  • • What is the specific plan for my baby's calcium levels immediately after birth to watch for transient hypocalcemia?
  • • Can you refer me to a genetic counselor who has experience with FHH and calcium-sensing disorders?

Questions for You

  • • How do I feel about the 50% chance of passing FHH1 to my children, knowing it is generally a benign condition?
  • • Do I know the calcium history of my partner's family? (This helps clarify the risk of more severe neonatal conditions).
  • • If I am currently pregnant, have I shared my FHH1 diagnosis with my obstetrician and the pediatrician who will see my baby?

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References

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    Prenatal diagnosis 2022; (42(5)):583-588 doi:10.1002/pd.6134.

    PMID: 35301736
  2. 2

    Case Report: Severe Neonatal Course in Paternally Derived Familial Hypocalciuric Hypercalcemia.

    Höppner J, Lais S, Roll C, et al.

    Frontiers in endocrinology 2021; (12()):700612 doi:10.3389/fendo.2021.700612.

    PMID: 34659108
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    Familial hypocalciuric hypercalcemia in an infant: diagnosis and management quandaries.

    Goldsweig B, Turk Yilmaz RS, Ravindranath Waikar A, et al.

    Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research 2024; (39(10)):1406-1411 doi:10.1093/jbmr/zjae137.

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    [A series of clinical cases of familial hypocalciuric hypercalcemia syndrome].

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    PMID: 33369373
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    Endocrine 2021; (71(3)):611-617 doi:10.1007/s12020-021-02620-5.

    PMID: 33528764
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    Identification of a novel large CASR deletion in a patient with familial hypocalciuric hypercalcemia.

    García-Castaño A, Madariaga L, Azriel S, et al.

    Endocrinology, diabetes & metabolism case reports 2018; (2018()).

    PMID: 30530875
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    Demineralization of Osseous Structures as Presentation of a Rare Genetic Disorder That Is Associated With a High Rate of Mortality.

    Afrah A, Finkel MA, Fonseca C, et al.

    Case reports in endocrinology 2024; (2024()):6063059 doi:10.1155/crie/6063059.

    PMID: 39703927
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    The Biochemical Profile of Familial Hypocalciuric Hypercalcemia and Primary Hyperparathyroidism during Pregnancy and Lactation: Two Case Reports and Review of the Literature.

    Ghaznavi SA, Saad NM, Donovan LE

    Case reports in endocrinology 2016; (2016()):2725486 doi:10.1155/2016/2725486.

    PMID: 27957351
  9. 9

    Familial Hypocalciuric Hypercalcemia in Pregnancy: Diagnostic Pitfalls.

    Jones AR, Hare MJ, Brown J, et al.

    JBMR plus 2020; (4(6)):e10362 doi:10.1002/jbm4.10362.

    PMID: 32537548
  10. 10

    Neonatal Hypocalcemic Seizures in Offspring of a Mother With Familial Hypocalciuric Hypercalcemia Type 1 (FHH1).

    Dharmaraj P, Gorvin CM, Soni A, et al.

    The Journal of clinical endocrinology and metabolism 2020; (105(5)) doi:10.1210/clinem/dgaa111.

    PMID: 32150253
  11. 11

    Neonatal hypocalcemia caused by maternal hypercalcemia: clinical characteristics, etiology, treatment, and outcome.

    Ding JJ, Chiu CF, Su YT, et al.

    Pediatric research 2025; doi:10.1038/s41390-025-04577-x.

    PMID: 41353515
  12. 12

    Reduced affinity of calcium sensing-receptor heterodimers and reduced mutant homodimer trafficking combine to impair function in a model of familial hypocalciuric hypercalcemia type 1.

    Wang X, Lundblad J, Smith SM

    PloS one 2022; (17(7)):e0266993 doi:10.1371/journal.pone.0266993.

    PMID: 35857775
  13. 13

    Heterozygous inactivating CaSR mutations causing neonatal hyperparathyroidism: function, inheritance and phenotype.

    Glaudo M, Letz S, Quinkler M, et al.

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    PMID: 27666534
  14. 14

    Genetics of hereditary forms of primary hyperparathyroidism.

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  15. 15

    Novel mutations of the calcium-sensing receptor impede differential diagnosis of primary hyperparathyroidism and familial hypocalciuric hypercalcemia.

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    PMID: 35242665

This page provides educational information about FHH1, genetics, and pregnancy. Always consult your obstetrician, endocrinologist, or a genetic counselor for personalized family planning and prenatal medical advice.

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