Mothers-to-be who suffer from the same severe morning sickness that the Duchess of Cambridge was treated for have an increased risk of a number of complications if they have the condition during the second trimester of pregnancy, research suggests.
There is a higher risk of preterm pre-eclampsia, premature separation of the placenta or having a small baby if hyperemesis gravidarum is onset during the second trimester of pregnancy, researchers said.
In December Kate was admitted to King Edward VII Hospital suffering from the condition - a rare illness which causes severe vomiting during pregnancy.
The condition is most common in the first 12 weeks of pregnancy and often eases off in the middle trimester.
St James' Palace confirmed a few weeks ago that Kate, whose baby is due in July, is continuing to feel better.
New research out today suggests that having the condition during the second trimester can lead to some placental complications.
The study, published in BJOG: An International Journal of Obstetrics and Gynaecology, examined data from the Swedish Medical Birth Register between 1997 and 2009.
Data from 1,155,033 women showed that 1.1% of them suffered from hyperemesis gravidarum before they were 22 weeks pregnant.
Researchers compared data concerning normal pregnancies to women who suffered from hyperemesis gravidarum in the second trimester - or women who were admitted to hospital with the condition between 12 and 21 weeks pregnancy.
Women who had the condition in the second trimester had a doubled risk of preterm pre-eclampsia - 0.6% of women without hyperemesis gravidarum suffered from the condition compared to 1.4% of women who did.
They also had a threefold risk of placental abruption - 0.4% compared to 1.1% - and a 39% increased risk of having a small baby.
Researchers also found a slightly increased risk of pre-eclampsia in women who had hyperemesis gravidarum in the first trimester - those who were admitted to hospital before they were 12 weeks pregnant.
Co-author of the study Marie Bolin, of the Department of Women's and Children's Health at Uppsala University in Sweden, said: "Our study found clear associations in the risk of pre-eclampsia, placental abruption and SGA (small for gestational age babies) birth in women presenting with hyperemesis gravidarum, particularly those presenting in the second trimester.
"The results indicate that pregnancies with hyperemesis gravidarum in the second trimester demand an increased alertness and supervision during the pregnancy for development of any adverse outcomes associated with abnormal placentation.
"Further research is needed to consider the best treatment and techniques for surveying blood pressure and fetal growth in these high risk women."
John Thorp, BJOG deputy editor in chief, added: "While the findings of this large study are compelling with strong associations found between hyperemesis gravidarum in the second trimester and placental dysfunction disorders, hyperemesis gravidarum remains a rare condition.
"As shown in the study, the time of onset of hyperemesis gravidarum influences the risks of abnormal placentation disorders, therefore women presenting during their first trimester of pregnancy should not be concerned that they will go on to experience adverse complications.
"Women presenting in the second trimester should be more aware of the risks and consult with their obstetrician for any concerns."
Hyperemesis gravidarum occurs in 0.5 to 3% of pregnancies.
The condition is caused by high levels of the pregnancy hormone, human chorionic gonadotropin (hCG), and a woman's sensitivity to it.