Weight-loss surgery offers the most effective long-term treatment for people living with obesity and its related complications, such as type 2 diabetes.1 Of the metabolic bariatric surgery types, Roux-en-Y gastric bypass (RYGB) achieves the most significant and sustained weight loss.2
Around 80% of these procedures are performed on women, many of whom are of reproductive age.3 For many, one of the most profound outcomes of surgery is the restoration of fertility. This has, in turn, given rise to a new and rapidly growing cohort in our antenatal clinics: women presenting with pregnancy after weight-loss surgery (PaWS).
'For clinicians, this is not just a variation of a high-risk pregnancy; it is an entirely new frontier.'
This new patient population presents a distinct clinical paradox. On the one hand, PaWS is associated with welcome improvements in traditional obesity-related complications, such as hypertension and gestational diabetes (GDM). On the other, it introduces a new, challenging set of risks, including a higher prevalence of babies born small-for-gestational age (SGA), preterm delivery, and even neonatal death.4
For clinicians, this is not just a variation of a high-risk pregnancy; it is an entirely new frontier. It requires a fundamental shift in our grasp of the underlying mechanisms and has important implications for management.
MECHANISMS
Understanding why these new risks emerge is key. The exact surgical changes that instigate weight loss create a unique physiological environment for pregnancy.
Balancing nutrition
Bariatric procedures are designed to cause malabsorption of key nutrients. Although patients are on micronutrient supplementation, the nausea and vomiting common in early pregnancy can upset this delicate balance. This can quickly lead to significant deficiencies in iron, vitamin B12 and vitamin D, all of which are critical for both maternal health and fetal development.
A battle for resources
The timing of pregnancy is critical. The first 12 months post-surgery are a period of rapid weight loss – a catabolic state. By contrast, pregnancy – particularly early gestation – is an anabolic state.5 In a woman who conceives within the first 12 months of surgery, these two opposing metabolic drives compete, often resulting in reduced gestational weight gain and restricted fetal growth. The data are striking: the overall risk of a baby born SGA in PaWS is 23%, but this risk leaps to 31% if conception occurs within the first year.6
Post-bariatric hypoglycaemia
'This extreme glycaemic variability makes the standard oral glucose tolerance test (OGTT) for GDM not just non-diagnostic, but actively dangerous.'
For many women, especially after RYGB, eating high-glycaemic index foods triggers a cascade. The altered gut anatomy contributes to changes in both gut hormones and glucose dynamics. There is accelerated glucose absorption with subsequent rapid, substantial, insulin secretion. This, in turn, can cause a ‘crash’, or severe post-prandial hypoglycaemia. This extreme glycaemic variability makes the standard oral glucose tolerance test (OGTT) for GDM not just non-diagnostic, but actively dangerous.
MANAGEMENT
Given these unique mechanisms, PaWS management must be proactive, multidisciplinary and evidence-based.
Collaboration with obstetric colleagues
These pregnancies should be managed as ‘high-risk’ pregnancies, and therefore it is critical that there is clear and frequent communication with supervising obstetric colleagues.
Vigilant nutritional surveillance
Offer nutritional monitoring with blood tests in each trimester as per British Obesity and Metabolic Specialist Society guidance.7
• Blood tests: ferritin, folate, vitamin B12, calcium (bone profile) and vitamins D and A. Those with long-limbed bypass or biliopancreatic diversion/duodenal switch also need monitoring of vitamins E and K.
• Supplementation: ensure women are on adequate multivitamins, vitamin D, calcium, folic acid, vitamin B12 and iron as needed.
Rethinking screening for GDM
Despite a higher risk of GDM in this cohort, the OGTT can trigger severe hypoglycaemia. The consensus recommendation is to screen at 24–28 weeks by self-monitoring of blood glucose or continuous glucose monitoring.
Managing the ‘crash’
For women experiencing post-bariatric hypoglycaemia, specialist dietetic advice is essential. This includes small, frequent meals, omitting fluid intake 30 minutes prior to and after eating, increasing protein intake, and strict avoidance of high-glycaemic index foods.
Fetal growth monitoring
' We must be able to counsel women not only about the risks to their pregnancy but also about the potential lasting health outcomes for their children.'
Given the high risk of SGA, clinicians should offer increased fetal growth monitoring, especially in GDM or if maternal gestational weight gain is inadequate.
Postpartum care
The need for supplementation does not end at delivery. Women must be supported with adequate micronutrients during breastfeeding, and reminded to continue lifelong multivitamins.
UNANSWERED QUESTIONS
While we have a strategy for managing the pregnancy itself, we are only just beginning to understand the longer-term effects in this cohort. The most pressing questions are no longer just about the mother, but also about the child. What are the effects of this unique in utero environment – one characterised by potential micronutrient deficiency and, crucially, extreme glycaemic variability – on a child’s future cardiometabolic and neurocognitive development?
Studies across the world are following up women with PaWS longitudinally, to explore what these alterations in their physiology mean for both maternal health and the long-term development of the child. This research is vital. We must be able to counsel women not only about the risks to their pregnancy but also about the potential lasting health outcomes for their children. As bariatric surgery becomes more common, the PaWS population is growing. It is our responsibility to fill these gaps in our knowledge, to ensure the best possible start in life for the next generation.
RAJ VALAIYAPATHI AND TRICIA TAN
Department of Metabolism, Digestion and Reproduction, Imperial College London
REFERENCES
1. Mingrone G et al. 2021 Lancet https://doi.org/10.1016/S0140-6736(20)32649-0.
2. By-Band-Sleeve Collaborative Group 2025 Lancet Diabetes & Endocrinology https://doi.org/10.1016/S2213-8587(25)00025-7.
3. UK National Bariatric Surgery Registry 2020 Third Registry Report p 27 https://e-dendrite.com/Publishing/Reports/Bariatric/NBSR2020.pdf.
4. Akhter Z et al. 2019 PLoS Medicine https://doi.org/10.1371/journal.pmed.1002866.
5. Lain KY & Catalano PM 2007 Clinical Obstetrics & Gynecology https://doi.org/10.1097/GRF.0b013e31815a5494.
6. Heusschen L et al. 2021 Obesity Surgery https://doi.org/10.1007/s11695-020-05219-3.
7. Shawe J et al. 2019 Obesity Reviews https://doi.org/10.1111/obr.12927.
